Bring back gc() function

Renaming gc() to _gc() was a mistake since the better thing to do is put
it behind the _COSMO_SOURCE macro. We need this change because I haven't
wanted to use my amazing garbage collector ever since we renamed it. You
now need to define _COSMO_SOURCE yourself when using amalgamation header
and cosmocc users need to pass the -mcosmo flag to get the gc() function

Some other issues relating to cancelation have been fixed along the way.
We're also now putting cosmocc in a folder named `.cosmocc` so it can be
more safely excluded by grep --exclude-dir=.cosmocc --exclude-dir=o etc.

test/libc/calls/poll_test.c

@@ -96,7 +96,7 @@ TEST(poll, testNegativeOneFd_isIgnored) {
   EXPECT_SYS(0, 0, poll(fds, ARRAYLEN(fds), 1));
   EXPECT_STREQ("fd:-1 revents:<TODO:kPollNames>\n"
                "fd:3 revents:<TODO:kPollNames>\n",
-               _gc(FormatPollFd(&fds[0])));
+               gc(FormatPollFd(&fds[0])));
   ASSERT_SYS(0, 0, close(3));
 }