[PowerPC] fix broken JIT-compiled code with tail call optimization

The relocation for branch instructions in the dynamic loader of ExecutionEngine assumes branch instructions with R_PPC64_REL24 relocation type are only bl. However, with the tail call optimization, b instructions can be also used to jump into another function.
This patch makes the relocation to keep bits in the branch instruction other than the jump offset to avoid relocation rewrites a b instruction into bl.

Differential Revision: https://reviews.llvm.org/D47456

llvm-svn: 333502