MIPS16: Mark $2/$3 as clobbered if GP is used

MIPS16: Mark $2/$3 as clobbered if GP is used

PR Target/84790.
The gp init sequence
        li      $2,%hi(_gp_disp)
        addiu   $3,$pc,%lo(_gp_disp)
        sll     $2,16
        addu    $2,$3
is generated directly in `mips_output_function_prologue`, and does
not appear in the RTL.

So the IRA/IPA passes are not aware that $2/$3 have been clobbered,
so they may be used for cross (local) function call.

Let's mark $2/$3 clobber both:
  - Just after the UNSPEC_GP RTL of a function;
  - Just after a function call.

Reported-by: Matthias Schiffer <mschiffer@universe-factory.net>
Origin-Patch-by: Felix Fietkau <nbd@nbd.name>.
gcc
	* config/mips/mips.cc(mips16_gp_pseudo_reg): Mark
	MIPS16_PIC_TEMP and MIPS_PROLOGUE_TEMP clobbered.
	(mips_emit_call_insn): Mark MIPS16_PIC_TEMP and
	MIPS_PROLOGUE_TEMP clobbered if MIPS16 and CALL_CLOBBERED_GP.

(cherry picked from commit 915440eed21de367cb41857afb5273aff5bcb737)