runtime: tolerate vendor suffixes in Linux kernel release strings

Synology kernels can have a "_" in their uname version. Make the
parsing more tolerant. And also don't throw during init if we fail to
parse the kernel version. Instead, fall back to probing. (We can't
probe all the time, because seccomp filters on some platforms like
Android kill the process if we call a verbotenen system call)

This regressed in CL 758902 (forked from CL 751340) on 2026-03-24,
which started calling parseRelease unconditionally during osinit on
32-bit Linux.

Updates #79612

Change-Id: I98f61b94e54c7b9d08029f3aef664bdda9ec7f69
Reviewed-on: https://go-review.googlesource.com/c/go/+/781800
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>