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.