runtime: keep //go:cgo_unsafe_args arguments alive to prevent GC

When syscall's DLL.FindProc calls into syscall_getprocaddress with a
byte slice pointer, we need to keep those bytes alive. Otherwise the GC
will collect the allocation, and we wind up calling `GetProcAddress` on
garbage, which showed up as various flakes in the builders. It turns out
that this problem extends to many uses of //go:cgo_unsafe_args
throughout, on all platforms. So this patch fixes the issue by keeping
non-integer pointer arguments alive through their invocation in
//go:cgo_unsafe_args functions.

Fixes #49731.

Change-Id: I93e4fbc2e8e210cb3fc53149708758bb33f2f9c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/367654


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Patrik Nyblom <pnyb@google.com>
TryBot-Result: Go Bot <gobot@golang.org>