runtime: portable access to sigev_notify_thread_id

Previously, libgo relied on the _sigev_un implementation-specific
field in struct sigevent, which is only available on glibc.
This patch uses the sigev_notify_thread_id macro instead which is
mandated by timer_create(2). In theory, this should work with any libc
implementation for Linux. Unfortunately, there is an open glibc bug
as glibc does not define this macro. For this reason, a glibc-specific
workaround is required. Other libcs (such as musl) define the macro
and don't require the workaround.

See https://sourceware.org/bugzilla/show_bug.cgi?id=27417

This makes libgo compatible with musl libc.

Based on patch by Sören Tempel.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/434755

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