mips: Add support for PC32 relocations in vmlinux

MIPS supports PC32 relocations like most other architectures, which will
be used by kallsyms to make its symbol references visible to the linker.

Given that these are place-relative, they can be ignored by the 'relocs'
tool, just like other PC type relocations.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>