[mac/lld] Fix scale computation for vector ops in PAGEOFF12 relocations

With this, llvm-tblgen no longer tries and fails to allocate 7953 petabyte
when it runs during the build. Instead, `check-llvm` with lld/mac as host
linker now completes without any failures on an m1 mac.

This vector op handling code matches what happens in:
- ld64's OutputFile::applyFixUps() in OutputFile.cpp for kindStoreARM64PageOff12
- lld.ld64.darwinold's offset12KindFromInstruction() in
  lld/lib/ReaderWriter/MachO/ArchHandler_arm64.cpp for offset12scale16
- RuntimeDyld's decodeAddend() in
  llvm/lib/ExecutionEngine/RuntimeDyld/Targets/RuntimeDyldMachOAArch64.h for
  ARM64_RELOC_PAGEOFF12

Fixes PR49444.

Differential Revision: https://reviews.llvm.org/D98053