aarch64: Update target checks for sme2 fp8

aarch64: Update target checks for sme2 fp8

Commits gcc-16-6381-g226d5fd59dc8 and gcc-16-6380-gef533d234293 had
insufficient target checks and this caused regressions on the linaro CI
which uses an older binutils version.

This change adds the needed checks.

gcc/testsuite/Changelog:
	* gcc.target/aarch64/sme2/acle-asm/cvt_mf8_bf16_x2.c: Added target checks.
	* gcc.target/aarch64/sme2/acle-asm/cvt_mf8_f16_x2.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/cvt_mf8_f32_x4.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/cvtn_mf8_f32_x4.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/scale_f16_x2.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/scale_f16_x4.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/scale_f32_x2.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/scale_f32_x4.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/scale_f64_x2.c: Likewise.
	* gcc.target/aarch64/sme2/acle-asm/scale_f64_x4.c: Likewise.