[efi] Ignore new LoongArch PC-relative relocations and relaxations

[efi] Ignore new LoongArch PC-relative relocations and relaxations

Several new relocations types have been added in LoongArch ABI version
2.10.  In particular:

- R_LARCH_B16 (18-bit PC-relative jump)
- R_LARCH_B21 (23-bit PC-relative jump)
- R_LARCH_PCREL20_S2 (22-bit PC-relative offset)

Also relocation relaxations have been introduced.  Recent GCC (13.2)
and binutils 2.41+ use these types of relocations, which confuses
elf2efi tool.  As a result, iPXE EFI images for LoongArch fail to
build with the following error:

  Unrecognised relocation type 103

Fix by ignoring R_LARCH_B{16,21} and R_LARCH_PCREL20_S2 (as with other
PC-relative relocations), and by ignoring relaxations (R_LARCH_RELAX).
Relocation relaxations are basically optimizations: ignoring them
results in a correct binary (although it might be suboptimal).

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>