[RuntimeDyld][ORC] Add support for Thumb mode to RuntimeDyldMachOARM.

This patch adds support for thumb relocations to RuntimeDyldMachOARM, and adds
a target-specific flags field to JITSymbolFlags (so that on ARM we can record
whether each symbol is Thumb-mode code).

RuntimeDyldImpl::emitSection is modified to ensure that stubs memory is
correctly aligned based on the size returned by getStubAlignment().

llvm-svn: 310517