Invalidate instruction cache when setting memory to be executable.

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