[libc] Check signs instead of values in memcmp unittests.

The C standard only guarantees the sign of return value. The exact return
value is implementation defined.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D109588

GitOrigin-RevId: 2fc1913505e3d5f48b3d5d84ce372a34e661fc7e
1 file changed