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