[ARM] add Thumb-1 8-bit movs/adds relocations to LLVM

This patch adds the LLVM-side plumbing for the following relocations:
- R_ARM_THM_ALU_ABS_G0_NC
- R_ARM_THM_ALU_ABS_G1_NC
- R_ARM_THM_ALU_ABS_G2_NC
- R_ARM_THM_ALU_ABS_G3

(see section 5.6.1.5, Static Thumb16 relocations, of the AArch32 ELF Arm ABI:
https://github.com/ARM-software/abi-aa/blob/844a79fd4c77252a11342709e3b27b2c9f590cf1/aaelf32/aaelf32.rst#5615static-thumb16-relocations)

Which can respectivly be generated by prefixing assembly symbols with:
- :lower0_7:
- :lower8_15:
- :upper0_7:
- :upper8_15:

LLD support for these relocations will be added in a follow-up patch

Reviewed By: john.brawn, MaskRay

Differential Revision: https://reviews.llvm.org/D149443