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

[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().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310517 91177308-0d34-0410-b5e6-96231b3b80d8