process-attributes uses /proc/stat for boot time

process-attributes uses /proc/stat for boot time

With a Linux kernel, use /proc/stat rather than get_boot_time
to get boot time, since when Emacs is running in a container
we want the underlying host boot time, not the container boot time.
This should be a better way to fix Bug#63496.
* src/sysdep.c: Do not include boot-time.h.
(get_host_boot_time) [GNU_LINUX || CYGWIN || __ANDROID__]:
New function.
(system_process_attributes) [GNU_LINUX || CYGWIN || __ANDROID__]:
Use it instead of get_boot_time.

(cherry picked from commit 73cebbb87b72effb948f0d73e756a0dc0c55b71c)