[ARM][test] Test .long thumb_func - .

Commit 9746286b from 2011 is related to
a subtle case that is untested. GNU assembler produces an R_ARM_REL32
relocation.
We currently don't, therefore we have to add a workaround in MCExpr.cpp
to make the value odd (see commit 8ee6c0ea "[MC] Remove isThumbFunc special case").