[Mips] Use appropriate private label prefix based on Mips ABI

MipsMCAsmInfo was using '$' prefix for Mips32 and '.L' for Mips64
regardless of -target-abi option. By passing MCTargetOptions to MCAsmInfo
we can find out Mips ABI and pick appropriate prefix.

Tags: #llvm, #clang, #lldb

Differential Revision: https://reviews.llvm.org/D66795