[lld][ELF] Allow Arm PC-relative relocations in PIC links (#77304)

The relocations that map to R_ARM_PCA are equivalent to R_PC. They are
PC-relative and safe to use in shared libraries, but have a different
relocation code as they are evaluated differently. Now that LLVM may
generate these relocations in object files, they may occur in
shared libraries or position-independent executables.