lib/libc: Provide optional check for time_t size

The LIBC_ALLOW_LESS_THAN_64BIT_TIME option disables a compile-time check
ensuring that time_t can hold 64-bit values. This check can prevent
accidental builds on targets with 32-bit time_t and the consequent 2038
issues.

This option is enabled when using the native C library as i386 glibc
uses 32-bit time_t.

A new file, validate_libc.c, is added to lib/libc to contain this and any
future libc tests. The check for CONFIG_EXTERNAL_LIBC is moved to
libc/CMakeLists.txt to ensure that this new file is always compiled.

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