Fix gcc build

We have -Werror=strict-overflow so gcc complains:

In file included from /tmp/obj/workspace/src/amd64.amd64/tmp/usr/include/bitstring.h:36:0,
                 from /workspace/src/tests/sys/sys/bitstring_test.c:34:
/workspace/src/tests/sys/sys/bitstring_test.c: In function 'bit_ffc_at_test':
/workspace/src/sys/sys/bitstring.h:239:5: error: assuming signed overflow does not occur when assuming that (X + c) >= X is always true [-Werror=strict-overflow]
  if (_start >= _nbits) {
     ^

Disable assuming overflow of signed integer will never happen by specifying
-fno-strict-overflow

Sponsored by:	The FreeBSD Foundation

tests/sys/sys/Makefile

@@ -1,9 +1,15 @@
 # $FreeBSD$
 
+.include <bsd.compiler.mk>
+
 TESTSDIR=	${TESTSBASE}/sys/sys
 
 ATF_TESTS_C=	arb_test bitstring_test qmath_test rb_test splay_test
 
 WARNS?=	5
 
+.if ${COMPILER_TYPE} == "gcc"
+CFLAGS.bitstring_test=	-fno-strict-overflow
+.endif
+
 .include <bsd.test.mk>