MIPS: tools: relocs: Ship a definition of R_MIPS_PC32

MIPS: tools: relocs: Ship a definition of R_MIPS_PC32

R_MIPS_PC32 is a GNU extension, its definition is available in glibc
only since 2.39 (released in 2024), and not available in musl libc yet.
Provide our own definition for R_MIPS_PC32 and use it if necessary to
fix relocs tool building on musl and older glibc systems.

Fixes: ff79d31eb536 ("mips: Add support for PC32 relocations in vmlinux")
Signed-off-by: Yao Zi <me@ziyao.cc>
Link: https://patch.msgid.link/20260202041610.61389-1-me@ziyao.cc
Signed-off-by: Nathan Chancellor <nathan@kernel.org>