fix(libc): fmod leaves errno unchanged

fmod set errno (33) for 1.0, 0.0. FreeBSD msun never sets errno and
reports errors through floating-point exceptions only, so errno is now
saved and restored around the host call.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

core/libs/prx/libc/src/Math.cpp

@@ -127,7 +127,12 @@ float APS5_VABI logf_nid_postfix(float x) { return std::log(x); }
 float APS5_VABI log2f_nid_postfix(float x) { return std::log2(x); }
 double APS5_VABI log10_nid_postfix(double x) { return std::log10(x); }
 float APS5_VABI ldexpf_nid_postfix(float x, int exp) { return std::ldexp(x, exp); }
-double APS5_VABI fmod_nid_postfix(double x, double y) { return std::fmod(x, y); }
+double APS5_VABI fmod_nid_postfix(double x, double y) {
+    int saved_errno = errno;
+    double result = std::fmod(x, y);
+    errno = saved_errno;
+    return result;
+}
 float APS5_VABI roundf_nid_postfix(float x) { return std::round(x); }
 double APS5_VABI round_nid_postfix(double x) { return std::round(x); }
 float APS5_VABI cbrtf_nid_postfix(float x) { return std::cbrt(x); }

core/libs/tests/GuestMath.cpp

@@ -38,6 +38,7 @@ extern LibcFloatConstant _FNan_nid_postfix;
 short APS5_VABI _FDtest_nid_postfix(const float*);
 int APS5_VABI __fpclassifyf_nid_postfix(float);
 float APS5_VABI fmodf_nid_postfix(float, float);
+double APS5_VABI fmod_nid_postfix(double, double);
 float APS5_VABI asinf_nid_postfix(float);
 float APS5_VABI acosf_nid_postfix(float);
 float APS5_VABI atan2f_nid_postfix(float, float);
@@ -150,6 +151,13 @@ static void CheckLongDouble() {
     Require(std::isnan(frexpl_nid_postfix(NAN, &exponent)) && exponent == 77);
 }
 
+static void CheckFmodErrno() {
+    errno = 0;
+    (void)fmod_nid_postfix(1.0, 0.0);
+    Require(errno == 0);
+    Require(fmod_nid_postfix(1e300, 3.0) == std::fmod(1e300, 3.0));
+}
+
 static void CheckIntegerConversions() {
     for (const long long numerator : {4294967301LL, -4294967301LL}) {
         for (const long long denominator : {3LL, -3LL}) {
@@ -493,4 +501,5 @@ int main() {
     Require(abs_nid_postfix(std::numeric_limits<int>::min()) == std::numeric_limits<int>::min());
     CheckHyperbolic();
     CheckHypotSpecialValue();
+    CheckFmodErrno();
 }