runtime/internal/sys: make Len64 nosplit
runtime/internal/sys: make Len64 nosplit

CL 427615 causes failures when Len64 can't be inlined. It's unclear to
me why this wasn't a problem before, but it is used in sensitive
contexts and therefore really should be marked. Confirmed that the
failures in question reproduce without this change, and don't reproduce
with it.

Fixes #55117.

Change-Id: Ic3aa96af1420cc0c39551908d83f954725c712f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/431058
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Auto-Submit: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go