Fix up PowerPC MMA flags so it builds by default.

Antonio Sánchezrequested to merge
cantonios/eigen:ppc into master

Introduces the build option

EIGEN_ALTIVEC_ENABLE_MMA_DYNAMIC_DISPATCH

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

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

Merge request reports