Optimize fmaxf etc.

[kripken]

The wasm builtins are very similar to the normal libc functions, except that nans are handled differently. Keep the musl nan handling, and otherwise use the builtins. This is ~41 bytes less in each fmaxf etc. function, and is 30% faster on this silly benchmark:

#include <math.h>
#include <stdio.h>

int main() {
  union {
    int i;
    float f;
  } u;
  float sum = 0;
  const int N = 20000;
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++) {
      u.i = ((i << 15) + j + 5085) ^ j ^ (i >> 2);
      sum += fmaxf(u.f, 0.5);
    }
  }
  printf("%.2f\n", sum);
}

After the speedup we are about equal with gcc natively.

Verified this does not change the output of our tests on this, and added more test coverage.

cc @sunfishcode

[sunfishcode]

Clever! I'm surprised by the magnitude of the speedup, but I guess that means those signbit calls aren't easy to optimize.

[jgravelle-google]

yay macros that make things easier to read :)

[kripken]

I added more tests for negative zero. Wasm and musl do handle it correctly even if the libc spec doesn't require it, so nice to make sure we don't regress that.