Use .text.unlikely and .text.eh prefixes for MachineBasicBlock sections.

Summary:
Instead of adding a ".unlikely" or ".eh" suffix for machine basic blocks,
this change updates the behaviour to use an appropriate prefix
instead. This allows lld to group basic block sections together
when -z,keep-text-section-prefix is specified and matches the behaviour
observed in gcc.

Reviewers: tmsriram, mtrofin, efriedma

Reviewed By: tmsriram, efriedma

Subscribers: eli.friedman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78742