generic: enable RELR relocations

generic: enable RELR relocations

RELR is a relocation packing format for relative relocations.

Linux has supported using it for ARM64 since 5.4, and more recently
for LoongArch as well.

It requires compatible tooling as well, so only with binutils 2.43
TOOLS_SUPPORT_RELR is set and thus RELR available.

RELR should always decrease the binary size and while testing this
the default build for Dynalink WRX-36 (qualcommax/ipq807x) reduced
the compressed kernel size by 38k and uncompressed one by 640k.

So, lets enable it in the default config.

Link: https://github.com/openwrt/openwrt/pull/16403
Signed-off-by: Robert Marko <robimarko@gmail.com>