process-attributes now uses get_boot_time

process-attributes now uses get_boot_time

I noticed this issue while looking into Bug#63496.
GNU Emacs does not infer the boot time consistently on GNU/Linux
and similar platforms: android_notifications_notify_1 and
get_boot_sec use Gnulib’s boot-time module, but process-attributes
has idiosyncratic code that evidently predates that module.
The idiosyncratic code returns an unstable etime, which is a minor bug.
Simplify process-attributes by just using Gnulib.
This returns a stable etime.
It may also fix Bug#63496; if not, we can fix Gnulib later.
* src/sysdep.c: Include <boot-time.h>.
(get_up_time) [GNU_LINUX || CYGWIN || __ANDROID__]: Remove.
(system_process_attributes) [GNU_LINUX || CYGWIN || __ANDROID__]:
Use get_boot_time instead of get_up_time.

(cherry picked from commit 477112ebc3497e9529cc06b24134d85d17c642ee)