include/pyconfig.h: Define `SIZEOF_PHTREAD_T` based on chosen libc

include/pyconfig.h: Define `SIZEOF_PHTREAD_T` based on chosen libc

The size of the `pthread_t` type varies between the libc/posix-threads
implementations, so we need to define the `SIZEOF_PTHREAD_T` macro
accordingly. This commit implements an additional
check for the Pthread Embedded library.

Signed-off-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #12