Invalidate instruction cache when setting memory to be executable.

lli's remote MCJIT code calls setExecutable just prior to running
code. In line with Darwin behaviour this seems to be the place to
invalidate any caches needed so that relocations can take effect
properly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182213 91177308-0d34-0410-b5e6-96231b3b80d8
AArch64: make RuntimeDyld relocations idempotent

AArch64 ELF uses .rela relocations so there's no need to actually make
use of the bits we're setting in the destination  However, we should
make sure all bits are cleared properly since multiple runs of
resolveRelocations are possible and these could combine to produce
invalid results if stale versions remain in the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182214 91177308-0d34-0410-b5e6-96231b3b80d8