[mach-o] Add Pass to create are shim Atoms for ARM interworking.

Arm code has two instruction encodings "thumb" and "arm".  When branching from
one code encoding to another, you need to use an instruction that switches
the instruction mode.  Usually the transition only happens at call sites, and
the linker can transform a BL instruction in BLX (or vice versa).  But if the
compiler did a tail call optimization and a function ends with a branch (not
branch and link), there is no pc-rel BX instruction.

The ShimPass looks for pc-rel B instructions that will need to switch mode.
For those cases it synthesizes a shim which does the transition, then modifies
the original atom with the B instruction to target to the shim atom.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219655 91177308-0d34-0410-b5e6-96231b3b80d8