avx: Fix incomplete naming of compiler options

Hi,

When testing Orc's bugreport tool, I noticed the following odd output:

Compiler options: sse2 sse3 ssse3 sse41 sse42 frame_pointer avx2 (null) (null)

Comparing with the SSE listing, there are two missing flags that I believed were not used. However, they are listed from within the CPUID test, and so an equivalent string constant is needed.

After fixing this, the output should read:

Compiler options: sse2 sse3 ssse3 sse41 sse42 sse4a 64bit avx avx2

which matches my Ryzen 7 2700x: