libzigc/math: Implement frexp

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=frexp -fqemu -fwasmtime --summary line
Build Summary: 737/737 steps succeeded
```

The tests were passing even when it was a straightforward calling of Zig
std library, but I wanted the `x is NaN` special case to match the
behaviour described in `libc` manpages, and for it to be consistent
with how infinities as arguments are handled in Zig.

lib/c/math.zig

@@ -60,6 +60,7 @@ comptime {
         symbol(&exp10, "exp10");
         symbol(&exp10f, "exp10f");
         symbol(&fdim, "fdim");
+        symbol(&frexp, "frexp");
         symbol(&hypot, "hypot");
         symbol(&modf, "modf");
         symbol(&pow, "pow");
@@ -161,6 +162,25 @@ fn fdim(x: f64, y: f64) callconv(.c) f64 {
     return 0;
 }
 
+fn frexp(x: f64, e: *c_int) callconv(.c) f64 {
+    // libc expects `*e` to be unspecified in this case; an unspecified C value
+    // should be a valid value of the relevant type, yet Zig's std
+    // implementation sets it to `undefined` -- which can even be nonsense
+    // according to the type (int). Therefore, we're setting it to a valid
+    // int value in Zig -- a zero.
+    //
+    // This mirrors the handling of infinities, where libc also expects
+    // unspecified for the value `*e` and Zig std sets it to a zero.
+    if (math.isNan(x)) {
+        e.* = 0;
+        return x;
+    }
+
+    const r = math.frexp(x);
+    e.* = r.exponent;
+    return r.significand;
+}
+
 fn hypot(x: f64, y: f64) callconv(.c) f64 {
     return math.hypot(x, y);
 }

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

@@ -1,23 +0,0 @@
-#include <math.h>
-#include <stdint.h>
-
-double frexp(double x, int *e)
-{
-	union { double d; uint64_t i; } y = { x };
-	int ee = y.i>>52 & 0x7ff;
-
-	if (!ee) {
-		if (x) {
-			x = frexp(x*0x1p64, e);
-			*e -= 64;
-		} else *e = 0;
-		return x;
-	} else if (ee == 0x7ff) {
-		return x;
-	}
-
-	*e = ee - 0x3fe;
-	y.i &= 0x800fffffffffffffull;
-	y.i |= 0x3fe0000000000000ull;
-	return y.d;
-}

src/libs/musl.zig

@@ -839,7 +839,6 @@ const src_files = [_][]const u8{
     "musl/src/math/__fpclassify.c",
     "musl/src/math/__fpclassifyf.c",
     "musl/src/math/__fpclassifyl.c",
-    "musl/src/math/frexp.c",
     "musl/src/math/frexpf.c",
     "musl/src/math/frexpl.c",
     "musl/src/math/i386/acosl.s",

src/libs/wasi_libc.zig

@@ -701,7 +701,6 @@ const libc_top_half_src_files = [_][]const u8{
     "musl/src/math/finitef.c",
     "musl/src/math/fma.c",
     "musl/src/math/fmaf.c",
-    "musl/src/math/frexp.c",
     "musl/src/math/frexpf.c",
     "musl/src/math/frexpl.c",
     "musl/src/math/ilogb.c",