JITEmitter.cpp was trying to sync the icache for function stubs, but

JITEmitter.cpp was trying to sync the icache for function stubs, but
was actually passing a completely incorrect size to sys_icache_invalidate.
Instead of having the JITEmitter do this (which doesn't have the correct
size), just make the target sync its own stubs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46354 91177308-0d34-0410-b5e6-96231b3b80d8