fixup! [libc][math] Optimize nearest integer functions using builtins when available

Fix builtin detection on Android due to missing _start being an error with LLD from Android NDK r26d.

Author: overmighty

libc/cmake/modules/compiler_features/check_builtin_ceil_floor_rint_trunc.cpp

@@ -8,4 +8,4 @@ double try_builtin_floor(double x) { return __builtin_floor(x); }
 double try_builtin_rint(double x) { return __builtin_rint(x); }
 double try_builtin_trunc(double x) { return __builtin_trunc(x); }
 
-int main() {}
+extern "C" void _start() {}

libc/cmake/modules/compiler_features/check_builtin_round.cpp

@@ -2,4 +2,4 @@ float try_builtin_roundf(float x) { return __builtin_roundf(x); }
 
 double try_builtin_round(double x) { return __builtin_round(x); }
 
-int main() {}
+extern "C" void _start() {}

libc/cmake/modules/compiler_features/check_builtin_roundeven.cpp

@@ -2,4 +2,4 @@ float try_builtin_roundevenf(float x) { return __builtin_roundevenf(x); }
 
 double try_builtin_roundeven(double x) { return __builtin_roundeven(x); }
 
-int main() {}
+extern "C" void _start() {}