Value of __ARM_ARCH_ISA_THUMB isn't based on the actual compilation
mode (-mthumb, -marm), it reflect's capability of given CPU.
Due to this:
•use tbumb and thumb2 insteand of __ARM_ARCH_ISA_THUMB
•use '.thumb' directive consistently in all affected files
•decorate all thumb functions using DEFINE_COMPILERRT_THUMB_FUNCTION()
(This is based off Michal's patch https://reviews.llvm.org/D30938)