Patch to add timegm POSIX call

[Andrew Russell via newlib]

Hi all:

I am proposing a patch (attached) which adds the timegm POSIX call to
Newlib. I tried to keep changes to a minimum, but part of this
refactors some of the code in libc/time/local.h and
libc/time/mktime.c, per this discussion:

https://sourceware.org/ml/newlib/2018/msg00186.html

Thanks,
Andrew