Explicitly turn on signed overflow semantics

Rakudo (if not Raku) assumes signed overflow, even though it's
technically undefined behavior in C. Add the '-fwrapv' flag, which AIUI
codified the behavior we expect in both GCC and Clang. This silences a
bunch of UBSAN warnings when running NQP tests.

Configure.pl

@@ -466,6 +466,7 @@ sub uniq {
 # generate CFLAGS
 my @cflags;
 push @cflags, '-std=c99' if $defaults{os} eq 'mingw32';
+push @cflags, '-fwrapv';
 push @cflags, $config{ccmiscflags};
 push @cflags, $config{ccoptiflags}  if $args{optimize};
 push @cflags, $config{ccdebugflags} if $args{debug};