PR47686. These micro-architecture levels are defined in the x86-64 psABI:
https://gitlab.com/x86-psABIs/x86-64-ABI/-/commit/77566eb03bc6a326811cb7e9
GCC 11 will support these levels.
Note, -mtune=x86-64-v[234] are invalid and __builtin_cpu_is cannot be
used on them.
Fix the bullet point: '-'
Please can you extend this and describe whats in each level? It'd be good to have this in a longer term doc as well as the release notes will get wiped at 13.0.