Fix up PowerPC MMA flags so it builds by default.

Introduces the build option

EIGEN_ALTIVEC_ENABLE_MMA_DYNAMIC_DISPATCH

If set and MMA builtins are available, will allow the dynamic dispatch path. If building for power10 and mma (-mmma -cpu=power10), switches to always use MMA.

Removed the EIGEN_ALTIVEC_DISABLE_MMA option. edit: put it back in.

Fixes #2457 (closed), and partly fixes #2324 (closed) in that the LTO issue should now be avoided by default.