mips: cps: Assemble jr.hb with an R2 ISA level

mips: cps: Assemble jr.hb with an R2 ISA level

A MIPS allmodconfig built with LLVM can select CPU_MIPS32_R1 together
with MIPS_MT_SMP. In that configuration clang invokes the integrated
assembler with -march=mips32, and the MIPS MT path in cps-vec.S fails
to assemble two jr.hb instructions:

  arch/mips/kernel/cps-vec.S:376:2: error: instruction requires
  a CPU feature not currently enabled

  arch/mips/kernel/cps-vec.S:490:4: error: instruction requires
  a CPU feature not currently enabled

The earlier jr.hb in the same file is already assembled inside a .set
MIPS_ISA_LEVEL_RAW scope. The two failing sites are reached after
popping back to the file's base ISA level, so LLVM correctly rejects
them for an R1 target.

Wrap those jr.hb instructions in the same ISA-level push/pop used by
the working site. This keeps the MT code unchanged while making the
required R2 hazard-branch encoding explicit to the assembler.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>