[ELF] Complete implementation of --fix-cortex-a53-843419

[ELF] Complete implementation of --fix-cortex-a53-843419

This patch provides the mechanism to fix instances of the instruction
sequence that may trigger the cortex-a53 843419 erratum. The fix is
provided by an alternative instruction sequence to remove one of the
erratum conditions. To reach this alternative instruction sequence we
replace the original instruction with a branch to the alternative
sequence. The alternative sequence is responsible for branching back to
the original.

As there is only erratum to fix the implementation is specific to
AArch64 and the specific erratum conditions. It should be generalizable
to other targets and erratum if needed.

Differential Revision: https://reviews.llvm.org/D36749

llvm-svn: 320800