testsuite: Require effective target bitint for test case

testsuite: Require effective target bitint for test case

Fix for the following test error on pru-unknown-elf:

  FAIL: gcc.dg/Wzero-as-null-pointer-constant-2.c (test for excess errors)
  Excess errors:
  .../gcc/gcc/testsuite/gcc.dg/Wzero-as-null-pointer-constant-2.c:9:19: sorry, unimplemented: '_BitInt(4)' is not supported on this target

gcc/testsuite/ChangeLog:

	* gcc.dg/Wzero-as-null-pointer-constant-2.c: Require effective
	target bitint.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>