LoongArch: Add DT_RELR support

The logic is same as a71d87680110 ("aarch64: Add DT_RELR support").

As LoongArch does not have -z dynamic-undefined-weak, we don't need to
consider UNDEFWEAK_NO_DYNAMIC_RELOC.

The linker relaxation adds another layer of complexity.  When we delete
bytes in a section during relaxation, we need to fix up the offset in
the to-be-packed relative relocations against this section.

Signed-off-by: Xi Ruoyao <xry111@xry111.site>