runtime: minor refactoring of _rt0_amd64_lib
runtime: minor refactoring of _rt0_amd64_lib

This function bounces between the C and Go ABIs a few times. This CL
narrows the scope of the Go -> C transition to just around the branch
that calls C. This lets us take advantage of C callee-save registers
to simplify the code a little.

Change-Id: I1ffa0b9e50325425c5ec66596978aeb6450a6b57
Reviewed-on: https://go-review.googlesource.com/c/go/+/309929
Trust: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

CAFxX/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,golang/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go