[ELF][RISCV] Allow R_RISCV_ADD in relocateNonAlloc()

gcc may generate .debug_info/.debug_aranges/.debug_line/etc that are
relocated by R_RISCV_ADD*/R_RISCV_SUB* pairs.
Allow R_RISCV_ADD in non-SHF_ALLOC section to fix link errors like:

    ld.lld: error: print.c:(.debug_frame+0x60): has non-ABS relocation R_RISCV_ADD64 against symbol '.L0 '

Differential Revision: https://reviews.llvm.org/D63259

llvm-svn: 365035