The R_ARM_ALU_PC_G0 and R_ARM_LDR_PC_G0 relocations are used by the ADR and LDR pseudo instructions, and are the basis of the group relocations that can load an arbitrary constant via a series of add, sub and ldr instructions.
The relocations need to be obtained via the .reloc directive.
R_ARM_ALU_PC_G0 is much more complicated as the add/sub instruction uses a modified immediate encoding of an 8-bit immediate rotated right by an even 4-bit field. This means that the range of representable immediates is sparse. We extract the encoding and decoding functions for the modified immediate from llvm/lib/Target/ARM/MCTargetDesc/ARMAddressingModes.h as this header file is not accessible from LLD. Duplication of code isn't ideal, but as these are well-defined mathematical functions they are unlikely to change.