disas/mips: Fix branch displacement for BEQZC and BNEZC

disas/mips: Fix branch displacement for BEQZC and BNEZC

disas/mips.c got added in commit 6643d27ea0 ("MIPS disas support")
apparently based on binutils tag 'gdb_6_1-branchpoint' [1].
Back then, MIPSr6 was not supported (added in binutils commit
7361da2c952 during 2014 [2]).

Binutils codebase diverged so much over the last 18 years, it is
not possible to simply cherry-pick their changes, so fix it BEQZC /
BNEZC 21-bit signed branch displacement locally.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=opcodes/mips-dis.c;hb=refs/tags/gdb_6_1-branchpoint
[2] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=7361da2c952

Fixes: 31837be3ee ("target-mips: add compact and CP1 branches")
Signed-off-by: David Daney <david.daney@fungible.com>
Reviewed-by: Marcin Nowakowski <marcin.nowakowski@fungible.com>
[PMD: Added commit description]
Signed-off-by: Philippe Mathieu-Daudé <philmd@fungible.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20221014112322.61119-1-philmd@fungible.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>