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. That 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.