instruction cache flushing in Cygwin

[Jeremy Drake]

I noticed while looking at JIT test failures in LLVM that they don't have
an implementation for Cygwin for flushing the instruction cache.  I don't
see any Cygwin function calling FlushInstructionCache, is there some
POSIXy function for this?  I don't really want to have to include Windows
headers just to get FlushInstructionCache and GetCurrentProcess, risking
pulling in a lot of extra definitions.