Put RTDyldMemoryManager into its own file, and make it linked into

libExecutionEngine. Move method implementations that aren't specific to 
allocation out of SectionMemoryManager and into RTDyldMemoryManager.

This is in preparation for exposing RTDyldMemoryManager through the C 
API.

This is a fixed version of r182407 and r182411. That first revision 
broke builds because I forgot to move the conditional includes of 
various POSIX headers from SectionMemoryManager into 
RTDyldMemoryManager. Those includes are necessary because of how 
getPointerToNamedFunction works around the glibc libc_nonshared.a thing. 
The latter revision still broke things because I forgot to include 
llvm/Config/config.h.

llvm-svn: 182418