[mips] Support 64-bit relative relocations

MIPS 64-bit ABI does not provide special PC-relative relocation like
R_MIPS_PC32 in 32-bit case. But we can use a "chain of relocation"
defined by N64 ABIs. In that case one relocation record might contain up
to three relocations which applied sequentially. Width of a final relocation
mask applied to the result of relocation depends on the last relocation
in the chain. In case of 64-bit PC-relative relocation we need the following
chain: `R_MIPS_PC32 | R_MIPS_64`. The first relocation calculates an
offset, but does not truncate the result. The second relocation just
apply calculated result as a 64-bit value.

The 64-bit PC-relative relocation might be useful in generation of
`.eh_frame` sections to escape passing `-Wl,-z,notext` flags to linker.

Differential Revision: https://reviews.llvm.org/D80390