Explicitly add -msse2 to compile HF related libgcc source file.

Explicitly add -msse2 to compile HF related libgcc source file.

For 32-bit libgcc configure w/o sse2, there's would be an error since
GCC only support _Float16 under sse2. Explicitly add -msse2 for those
HF related libgcc functions, so users can still link them w/ the
upper configuration.

libgcc/ChangeLog:

	* Makefile.in: Adjust to support specific CFLAGS for each
	libgcc source file.
	* config/i386/64/t-softfp: Explicitly add -msse2 for HF
	related libgcc source files.
	* config/i386/t-softfp: Ditto.
	* config/i386/_divhc3.c: New file.
	* config/i386/_mulhc3.c: New file.