enable sincos optimization on mingw-w64

enable sincos optimization on mingw-w64

The sincos transformation does not take place on all platforms,
because the libc_has_function target hook disables it by default.

Current mingw-w64's math library supports sincos, sincosl and sincosf,
in 32- and 64-bit modes.  I suppose this has been this way for long.

This patch enables the sincos optimization on this platform.

for  gcc/ChangeLog

	* config/i386/mingw-w64.h (TARGET_LIBC_HAS_FUNCTION): Enable
	sincos optimization.