[builtins][ARM] Select correct code fragments when compiling for Thumb1/Thum2/ARM ISA

Summary:
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)

Reviewers: dim, rengolin, compnerd, strejda

Reviewed By: compnerd

Subscribers: peter.smith, kubamracek, mgorny, javed.absar, kristof.beyls, jamesduley, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D31220

llvm-svn: 310884