gcc.target/i386/pr84876.c: Compile with -mno-avx512f

Compile gcc.target/i386/pr84876.c with -mno-avx512f to disable xmm16-xm31
registers to get the expected error:

../gcc.target/i386/pr84876.c:6:3: error: ‘asm’ operand has impossible constraints or there are not enough registers

when AVX512F is enabled by default.

	PR testsuite/126923
	* gcc.target/i386/pr84876.c (dg-options): Add -mno-avx512f.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>