gdb/testsuite: make gdb.reverse/i386-avx-reverse.exp require also avx2

The test gdb.reverse/i386-avx-reverse.exp requires CPU to have AVX
instructions but it actually also uses AVX2 instructions (like
vpcmpeqd). This caused the test to fail on CPUs that have AVX but not
AVX2.

This commit adds check for AVX2.

Tested on Intel Xeon CPU E3-1265L (no AVX2) and Intel Core i7-1355U
(has AVX2).
2 files changed