cmd/asm: when dynamic linking, reject code that uses a clobbered R15

The assember uses R15 as scratch space when assembling global variable
references in dynamically linked code. If the assembly code uses the
clobbered value of R15, report an error. The user is probably expecting
some other value in that register.

Getting rid of the R15 use isn't very practical (we could save a
register to a field in the G maybe, but that gets cumbersome).

Fixes #43661

Change-Id: I43f848a3d8b8a28931ec733386b85e6e9a42d8ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/283474


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>