libzigc: Implement tanh

The changes were tested by running following commands:

```
$ ./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=tanh -fqemu -fwasmtime --summary line
Build Summary: 1289/1289 steps succeeded
```

lib/c/math.zig

@@ -63,6 +63,7 @@ comptime {
         symbol(&pow, "pow");
         symbol(&pow10, "pow10");
         symbol(&pow10f, "pow10f");
+        symbol(&tanh, "tanh");
     }
 
     if (builtin.target.isMuslLibC()) {
@@ -338,3 +339,7 @@ test "rint" {
     try expectEqual(@as(f64, 2.0), rint(2.5));
     try expectEqual(@as(f64, 4.0), rint(3.5));
 }
+
+fn tanh(x: f64) callconv(.c) f64 {
+    return math.tanh(x);
+}

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

@@ -1,45 +0,0 @@
-#include "libm.h"
-
-/* tanh(x) = (exp(x) - exp(-x))/(exp(x) + exp(-x))
- *         = (exp(2*x) - 1)/(exp(2*x) - 1 + 2)
- *         = (1 - exp(-2*x))/(exp(-2*x) - 1 + 2)
- */
-double tanh(double x)
-{
-	union {double f; uint64_t i;} u = {.f = x};
-	uint32_t w;
-	int sign;
-	double_t t;
-
-	/* x = |x| */
-	sign = u.i >> 63;
-	u.i &= (uint64_t)-1/2;
-	x = u.f;
-	w = u.i >> 32;
-
-	if (w > 0x3fe193ea) {
-		/* |x| > log(3)/2 ~= 0.5493 or nan */
-		if (w > 0x40340000) {
-			/* |x| > 20 or nan */
-			/* note: this branch avoids raising overflow */
-			t = 1 - 0/x;
-		} else {
-			t = expm1(2*x);
-			t = 1 - 2/(t+2);
-		}
-	} else if (w > 0x3fd058ae) {
-		/* |x| > log(5/3)/2 ~= 0.2554 */
-		t = expm1(2*x);
-		t = t/(t+2);
-	} else if (w >= 0x00100000) {
-		/* |x| >= 0x1p-1022, up to 2ulp error in [0.1,0.2554] */
-		t = expm1(-2*x);
-		t = -t/(t+2);
-	} else {
-		/* |x| is subnormal */
-		/* note: the branch above would not raise underflow in [0x1p-1023,0x1p-1022) */
-		FORCE_EVAL((float)x);
-		t = x;
-	}
-	return sign ? -t : t;
-}

src/libs/musl.zig

@@ -1003,7 +1003,6 @@ const src_files = [_][]const u8{
     "musl/src/math/sinl.c",
     "musl/src/math/__tan.c",
     "musl/src/math/__tandf.c",
-    "musl/src/math/tanh.c",
     "musl/src/math/tanhf.c",
     "musl/src/math/tanhl.c",
     "musl/src/math/__tanl.c",

src/libs/wasi_libc.zig

@@ -793,7 +793,6 @@ const libc_top_half_src_files = [_][]const u8{
     "musl/src/math/sinl.c",
     "musl/src/math/__tan.c",
     "musl/src/math/__tandf.c",
-    "musl/src/math/tanh.c",
     "musl/src/math/tanhf.c",
     "musl/src/math/tanhl.c",
     "musl/src/math/__tanl.c",