MIPS: jump_label.c: Handle the microMIPS J instruction encoding

Implement the microMIPS encoding of the J instruction for the purpose of
the static keys feature, fixing a crash early on in bootstrap as the
kernel is unhappy seeing the ISA bit set in jump table entries.  Make
sure the ISA bit correctly reflects the instruction encoding chosen for
the kernel, 0 for the standard MIPS and 1 for the microMIPS encoding.

Also make sure the instruction to patch is a 32-bit NOP in the microMIPS
mode as by default the 16-bit short encoding is assumed

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8516/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>