lib: libc: default LIBC_ALLOW_LESS_THAN_64BIT_TIME for ARCMWDT

Set CONFIG_LIBC_ALLOW_LESS_THAN_64BIT_TIME to default 'y' for
ARCMWDT_LIBC toolchain.
The ARCMWDT_LIBC uses a 32-bit time_t
which fails the 64-bit time_t validation introduced to prevent Y2038
issues. This change eliminates the need to manually configure this
option for every ARCMWDT test while maintaining the validation
for other toolchains that support 64-bit time_t.

Signed-off-by: Mohamed Moawad <moawad@synopsys.com>