Supply a build rule for the recently added nptest

Commit d4ab70f2

 added a test program
to check that the NULL pointer is represented as all zero bits, but
did not specify a build rule for that new executable.  On many platforms,
the implicit rule sufficed, since nptest is a very simple program, but
for at least darwin-i386-cc, an explicit rule is needed.  On darwin-i386-cc,
the implicit rule targetted a 64-bit executable, but the object file
containing the definition of main was a 32-bit object, which the linker
excluded from consideration, resulting in a link failure due to no
definition for _main.

Add the missing build rule to fix the build on such platforms.

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Dr. Stephen Henson <steve@openssl.org>