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.

llvm-svn: 182411