cmd/link: use R12 as trampoline scratch register on ARM
cmd/link: use R12 as trampoline scratch register on ARM

The external linker uses R12. Do the same. We previously use R11,
the temp register in Go ABI. This does not really matter if the
caller is Go code, because all registers are clobbered at call.
But it the caller is C code, it may assume R11 live across a call.
Using R11 may clobber live value. On the callee side, R12 is not
an argument register in both Go and C calling convention.

Change-Id: I958c5dad52aa51bb282a7ad420f5423863e69c14
Reviewed-on: https://go-review.googlesource.com/c/go/+/314454
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

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