musl: improve crypt() size hack

Instead of silently downgrading any non-MD5 crypt() request to DES,
cleanly fail with return NULL and errno = ENOSYS. This allows callers
to notice the missing support instead of the unwanted silent fallback
to DES.

Also add a menuconfig toolchain option to optionally disable the crypt
size hack completely. This can be probably made dependant on SMALL_FLASH
or a similar feature indicator in a future commit.

Ref: https://github.com/openwrt/openwrt/pull/1331
Signed-off-by: Jo-Philipp Wich <jo@mein.io>