i386: Don't pass -msse2avx to assembler for -mavx

i386: Don't pass -msse2avx to assembler for -mavx

With

gcc -O2 -fPIC -flto -g -c -o a.o a.c
gcc -O2 -fPIC -flto -g -mavx   -c -o b.o b.c
gcc -shared -O2 -fPIC -flto -g -o lib1.so a.o b.o

LTO correctly generates AVX for b.o and SSE for a.o.  But the GCC driver
passes -msse2avx to assembler, which encodes SSE instructions as AVX
instructions.  We shouldn't pass -msse2avx to assembler for -mavx.

	PR target/87522
	* config/i386/gnu-user.h (ASM_SPEC): Don't pass -msse2avx to
	assembler for -mavx.
	* config/i386/gnu-user64.h (ASM_SPEC): Likewise.

From-SVN: r264864