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[MC][ELF][ARM] Add relocations for some pc-relative fixups

Authored by psmith on Feb 23 2020, 2:49 AM.

Description

[MC][ELF][ARM] Add relocations for some pc-relative fixups

Add ELF relocations for the following fixups:
fixup_thumb_adr_pcrel_10 -> R_ARM_THM_PC8
fixup_thumb_cp -> R_ARM_THM_PC8
fixup_t2_adr_pcrel_12 -> R_ARM_THM_PREL_11_0
fixup_t2_ldst_pcrel_12 -> R_ARM_THM_PC12

While these relocations are short-ranged there is support in the open
source ELF linker's in binutils and soon to be in LLD. MC will no longer
resolve pc-relative fixups to global symbols due to interpositioning
concerns. We can handle these at link time by implementing the relocations.

The R_ARM_THM_PC8 has some extra encoding rules for addends that llvm-mc
sidesteps by not supporting addends for these instructions, using the wide
Thumb 2 instruction if it is available. I think that this is a reasonable
compromise given that these are rare.

This partiall reverts D72892, the Thumb fixups no longer need to be
evaluated at assembly time.

Differential Revision: https://reviews.llvm.org/D75039