builtins: 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()

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Note: This patch doesn't fix broken Thumb1 variant of __udivsi3 !

Reviewers: weimingz, rengolin, compnerd

Subscribers: aemerson, dim

Differential Revision: https://reviews.llvm.org/D30938