tcg/ppc32: Use trampolines to trim the code size for mmu slow path accessors

mmu access looks something like:

<check tlb>
if miss goto slow_path
<fast path>
done:
...

; end of the TB
slow_path:
 <pre process>
 mr r3, r27         ; move areg0 to r3
                    ; (r3 holds the first argument for all the PPC32 ABIs)
 <call mmu_helper>
 b $+8
 .long done
 <post process>
 b done

On ppc32 <call mmu_helper> is:

(SysV and Darwin)

mmu_helper is most likely not within direct branching distance from
the call site, necessitating

a. moving 32 bit offset of mmu_helper into a GPR ; 8 bytes
b. moving GPR to CTR/LR                          ; 4 bytes
c. (finally) branching to CTR/LR                 ; 4 bytes

r3 setting              - 4 bytes
call                    - 16 bytes
dummy jump over retaddr - 4 bytes
embedded retaddr        - 4 bytes
         Total overhead - 28 bytes

(PowerOpen (AIX))
a. moving 32 bit offset of mmu_helper's TOC into a GPR1 ; 8 bytes
b. loading 32 bit function pointer into GPR2            ; 4 bytes
c. moving GPR2 to CTR/LR                                ; 4 bytes
d. loading 32 bit small area pointer into R2            ; 4 bytes
e. (finally) branching to CTR/LR                        ; 4 bytes

r3 setting              - 4 bytes
call                    - 24 bytes
dummy jump over retaddr - 4 bytes
embedded retaddr        - 4 bytes
         Total overhead - 36 bytes

Following is done to trim the code size of slow path sections:

In tcg_target_qemu_prologue trampolines are emitted that look like this:

trampoline:
mfspr r3, LR
addi  r3, 4
mtspr LR, r3      ; fixup LR to point over embedded retaddr
mr    r3, r27
<jump mmu_helper> ; tail call of sorts

And slow path becomes:

slow_path:
 <pre process>
 <call trampoline>
 .long done
 <post process>
 b done

call                    - 4 bytes (trampoline is within code gen buffer
                                   and most likely accessible via
                                   direct branch)
embedded retaddr        - 4 bytes
         Total overhead - 8 bytes

In the end the icache pressure is decreased by 20/28 bytes at the cost
of an extra jump to trampoline and adjusting LR (to skip over embedded
retaddr) once inside.

Signed-off-by: malc <av1474@comtv.ru>
