scripts/checkpatch: Check for patches adding #defines for libc APIs

All code in the Zephyr core must use only the Zephyr C library API
according to rules A.4 and A.5. Such code is not permitted to request API
extensions from the C library via any of the API request mechanisms.

This addition to checkpatch.pl verifies that patches don't #define
any of these:

	__STRICT_ANSI__
	_POSIX_SOURCE
	_POSIX_C_SOURCE
	_XOPEN_SOURCE
	_ISOC99_SOURCE
	_ISOC11_SOURCE
	_ATFILE_SOURCE
	_GNU_SOURCE
	_BSD_SOURCE
	_SVID_SOURCE
	_DEFAULT_SOURCE

Reference: #49922

Signed-off-by: Keith Packard <keithp@keithp.com>