posix: sys/stat.h: adopt the sys/stat.h header from picolibc

This may be getting ahead of the curve a bit, but here we adopt
the `<sys/stat.h>` header from picolibc. It is mostly the same
as that of newlib but the picolibc variant seemed to be slightly
more "inclusive".

Removed the `<_ansi.h>` include.

The commit immediately following this declares a few missing types
only if they are not already declared.

Signed-off-by: Chris Friedt <cfriedt@meta.com>