cmd/compile/abi-internal: declare X15 scratch in function bodies

X15 must be zero at function calls and returns, but can be used as
scratch in the middle of a function. This allows things like memmove
and the hashing functions to use X15 temporarily, as long as they set
it back to 0 before returning.

This CL also clarifies the distinction between register meanings on
function call versus function return, since some of them have fixed
meanings at both call and return, while others only have a fixed
meaning at calls.

Updates #40724.

Change-Id: I9dad3abde42cd4d2788e8435cde6d55073dd75a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/308929
Trust: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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