libzigc: Implement modfl

The changes were tested by running:

```
$ ./build/stage3/bin/zig build -p stage4 -Denable-llvm -Dno-lib

$ stage4/bin/zig build test-libc -Dlibc-test-path=<LIBC-TEST-PATH> -Dtest-filter=modfl -fqemu -fwasmtime --summary line
Build Summary: 369/369 steps succeeded
```

lib/c/math.zig

@@ -39,6 +39,7 @@ comptime {
         symbol(&hypotf, "hypotf");
         symbol(&hypotl, "hypotl");
         symbol(&modff, "modff");
+        symbol(&modfl, "modfl");
         symbol(&nan, "nan");
         symbol(&nanf, "nanf");
         symbol(&nanl, "nanl");
@@ -201,11 +202,16 @@ fn modff(x: f32, iptr: *f32) callconv(.c) f32 {
     return modfGeneric(f32, x, iptr);
 }
 
+fn modfl(x: c_longdouble, iptr: *c_longdouble) callconv(.c) c_longdouble {
+    return modfGeneric(c_longdouble, x, iptr);
+}
+
 fn testModf(comptime T: type) !void {
     // Choose the appropriate `modf` impl to test based on type
     const f = switch (T) {
-        f64 => modf,
         f32 => modff,
+        f64 => modf,
+        c_longdouble => modfl,
         else => @compileError("modf not implemented for " ++ @typeName(T)),
     };
 
@@ -248,8 +254,9 @@ fn testModf(comptime T: type) !void {
 }
 
 test "modf" {
-    try testModf(f64);
     try testModf(f32);
+    try testModf(f64);
+    try testModf(c_longdouble);
 }
 
 fn nan(_: [*:0]const c_char) callconv(.c) f64 {

lib/libc/mingw/math/modfl.c

@@ -1,41 +0,0 @@
-/**
- * This file has no copyright assigned and is placed in the Public Domain.
- * This file is part of the mingw-w64 runtime package.
- * No warranty is given; refer to the file DISCLAIMER.PD within this package.
- */
-#include <fenv.h>
-#include <math.h>
-#include <errno.h>
-
-long double
-modfl (long double value, long double* iptr)
-{
-  long double int_part = 0.0L;
-  /* truncate */
-#if (defined(_AMD64_) && !defined(_ARM64EC_)) || (defined(__x86_64__) && !defined(__arm64ec__))
-  asm volatile ("subq $8, %%rsp\n"
-    "fnstcw 4(%%rsp)\n"
-    "movzwl 4(%%rsp), %%eax\n"
-    "orb $12, %%ah\n"
-    "movw %%ax, (%%rsp)\n"
-    "fldcw (%%rsp)\n"
-    "frndint\n"
-    "fldcw 4(%%rsp)\n"
-    "addq $8, %%rsp\n" : "=t" (int_part) : "0" (value) : "eax"); /* round */
-#elif defined(_X86_) || defined(__i386__)
-  asm volatile ("push %%eax\n\tsubl $8, %%esp\n"
-    "fnstcw 4(%%esp)\n"
-    "movzwl 4(%%esp), %%eax\n"
-    "orb $12, %%ah\n"
-    "movw %%ax, (%%esp)\n"
-    "fldcw (%%esp)\n"
-    "frndint\n"
-    "fldcw 4(%%esp)\n"
-    "addl $8, %%esp\n\tpop %%eax\n" : "=t" (int_part) : "0" (value) : "eax"); /* round */
-#else
-  int_part = truncl(value);
-#endif
-  if (iptr)
-    *iptr = int_part;
-  return (isinf (value) ?  0.0L : value - int_part);
-}

lib/libc/musl/src/math/modfl.c

@@ -1,53 +0,0 @@
-#include "libm.h"
-
-#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
-long double modfl(long double x, long double *iptr)
-{
-	double d;
-	long double r;
-
-	r = modf(x, &d);
-	*iptr = d;
-	return r;
-}
-#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
-
-static const long double toint = 1/LDBL_EPSILON;
-
-long double modfl(long double x, long double *iptr)
-{
-	union ldshape u = {x};
-	int e = (u.i.se & 0x7fff) - 0x3fff;
-	int s = u.i.se >> 15;
-	long double absx;
-	long double y;
-
-	/* no fractional part */
-	if (e >= LDBL_MANT_DIG-1) {
-		*iptr = x;
-		if (isnan(x))
-			return x;
-		return s ? -0.0 : 0.0;
-	}
-
-	/* no integral part*/
-	if (e < 0) {
-		*iptr = s ? -0.0 : 0.0;
-		return x;
-	}
-
-	/* raises spurious inexact */
-	absx = s ? -x : x;
-	y = absx + toint - toint - absx;
-	if (y == 0) {
-		*iptr = x;
-		return s ? -0.0 : 0.0;
-	}
-	if (y > 0)
-		y -= 1;
-	if (s)
-		y = -y;
-	*iptr = x + y;
-	return -y;
-}
-#endif

src/libs/mingw.zig

@@ -619,7 +619,6 @@ const mingw32_generic_src = [_][]const u8{
     "math" ++ path.sep_str ++ "lgamma.c",
     "math" ++ path.sep_str ++ "lgammaf.c",
     "math" ++ path.sep_str ++ "lgammal.c",
-    "math" ++ path.sep_str ++ "modfl.c",
     "math" ++ path.sep_str ++ "powi.c",
     "math" ++ path.sep_str ++ "powif.c",
     "math" ++ path.sep_str ++ "powil.c",

src/libs/musl.zig

@@ -934,7 +934,6 @@ const src_files = [_][]const u8{
     "musl/src/math/__math_uflowf.c",
     "musl/src/math/__math_xflow.c",
     "musl/src/math/__math_xflowf.c",
-    "musl/src/math/modfl.c",
     "musl/src/math/nearbyint.c",
     "musl/src/math/nearbyintf.c",
     "musl/src/math/nearbyintl.c",

src/libs/wasi_libc.zig

@@ -755,7 +755,6 @@ const libc_top_half_src_files = [_][]const u8{
     "musl/src/math/__math_uflowf.c",
     "musl/src/math/__math_xflow.c",
     "musl/src/math/__math_xflowf.c",
-    "musl/src/math/modfl.c",
     "musl/src/math/nearbyintl.c",
     "musl/src/math/nextafter.c",
     "musl/src/math/nextafterf.c",