C11 timespec_get(), TIME_UTC

Hey guys,

I'm a big user and fan of Newlib and have really enjoyed working with it.
In addition to it being used in a bunch of the builds I target in CI, I'm
one of the maintainers of the KallistiOS indie/homebrew SDK for the Sega
Dreamcast, which has an epic amount of modern language and stdlib support
for C and C++ thanks to you guys.

Anyway, I just encountered something today that is missing that I can't
help but feel like should really be included within the lib: support for
the C11 timespec struct, timespec_get() function, and the TIME_UTC macro,
used to simply get the current UTC time in seconds + nanoseconds:
https://en.cppreference.com/w/c/chrono/TIME_UTC

Given the amount of non-standard POSIX time functionality that is
supported, the modern C++ chrono stuff, the rest of the C stdlib, etc, it
feels like this should fall within the scope of the project. It looks like
everybody else is just doing a tiny little wrapper for it around
clock_gettime(), so it's simple to implement as well... I also see several
threads on stack overflow with people confused as to why it's not available
to them, so I'm not the only one interested.

I've never submitted a patch for anything like this before and wanted to
run this by you guys to make sure it fall within NewLib's scope before I
was going to give it a shot. What do you guys think?

Anyway, thanks again, sorry for the verbosity.