[MC][ELF] Copy top 3 bits of st_other to .symver aliases

On PowerPC64 ELFv2 ABI, the top 3 bits of st_other encode the local
entry offset. A versioned symbol alias created by .symver should copy
the bits from the source symbol.

This partly fixes PR41048. A full fix needs tracking of .set assignments
and updating st_other fields when finish() is called, see D56586.

Patch by Alfredo Dal'Ava Júnior

Differential Revision: https://reviews.llvm.org/D59436

llvm-svn: 360442