c++filt: flush output after newline

Some tools spawn c++filt and pass it a single line at a time for
demangling. This is akin to r276689 for addr2line.

Sponsored by:	The FreeBSD Foundation

contrib/elftoolchain/cxxfilt/cxxfilt.c

@@ -188,6 +188,8 @@ main(int argc, char **argv)
 				if (c == EOF)
 					break;
 				putchar(c);
+				if (c == '\n')
+					fflush(stdout);
 			} else {
 				if ((size_t) p >= sizeof(buf) - 1)
 					warnx("buffer overflowed");