ReaderWriter: adjust ARM target addresses for exec

ARM NT assumes a THUMB only environment.  As such, any address that is detected
as residing in an executable section is adjusted to have its bottom bit set to
indicate THUMB in case of a mode exchange.

Although the testing here seems insufficient (missing the negative cases) the
existing test cases for the IMAGE_REL_ARM_{ADDR32,MOV32T} are relevant as they
ensure that we do not incorrectly set the bit.

llvm-svn: 225104