cmd/internal/obj/arm64: optimize function prologue/epilogue with STP/LDP

In function prologue and epilogue, we save and restore FP and LR
registers, and adjust RSP. The current instruction sequence is as
follow.

For frame size <= 240B,
  prologue:
    MOVD.W	R30, -offset(RSP)
    MOVD	R29, -8(RSP)
  epilogue:
    MOVD	-8(RSP), R29
    MOVD.P	offset(RSP), R30

For frame size > 240B,
  prologue:
    SUB		$offset, RSP, R27
    MOVD	R30, (R27)
    MOVD	R27, RSP
    MOVD	R29, -8(RSP)
  epilogue:
    MOVD	-8(RSP), R29
    MOVD	(RSP), R30
    ADD		$offset, RSP

Each sequence uses two load or store instructions, actually we can load
or store two registers with one LDP or STP instruction. This CL changes
the sequences as follow.

For frame size <= 496B,
  prologue:
    STP		(R29, R30), -(offset+8)(RSP)
    SUB		$offset, RSP, RSP
  epilogue:
    LDP		-8(RSP), (R29, R30)
    ADD		$offset, RSP, RSP

For frame size > 496B,
  prologue:
    SUB		$offset, RSP, R20
    STP		(R29, R30), -8(R20)
    MOVD	R20, RSP
  epilogue:
    LDP		-8(RSP), (R29, R30)
    ADD		$offset, RSP, RSP

Change-Id: Ia58af85fc81cce9b7c393dc38df43bffb203baad
Reviewed-on: https://go-review.googlesource.com/c/go/+/379075


Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Eric Fang <eric.fang@arm.com>
Run-TryBot: Eric Fang <eric.fang@arm.com>