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.

third_party/lua/ldo.c

@@ -86,14 +86,14 @@ asm(".include \"libc/disclaimer.inc\"");
 #elif defined(LUA_USE_POSIX)				/* }{ */
 
 /* in POSIX, try _longjmp/_setjmp (more efficient) */
-#define LUAI_THROW(L,c)		_gclongjmp((c)->b, 1)
+#define LUAI_THROW(L,c)		gclongjmp((c)->b, 1)
 #define LUAI_TRY(L,c,a)		if (_setjmp((c)->b) == 0) { a }
 #define luai_jmpbuf		jmp_buf
 
 #else							/* }{ */
 
 /* ISO C handling with long jumps */
-#define LUAI_THROW(L,c)		_gclongjmp((c)->b, 1)
+#define LUAI_THROW(L,c)		gclongjmp((c)->b, 1)
 #define LUAI_TRY(L,c,a)		if (setjmp((c)->b) == 0) { a }
 #define luai_jmpbuf		jmp_buf
 

third_party/lua/ltests.c

@@ -1262,7 +1262,7 @@ static int panicback (lua_State *L) {
   b = (struct Aux *)lua_touserdata(L, -1);
   lua_pop(L, 1);  /* remove 'Aux' struct */
   runC(b->L, L, b->paniccode);  /* run optional panic code */
-  _gclongjmp(b->jb, 1);
+  gclongjmp(b->jb, 1);
   return 1;  /* to avoid warnings */
 }
 

third_party/lua/luaencodejsondata.c

@@ -22,7 +22,7 @@
 #include "libc/intrin/likely.h"
 #include "libc/log/log.h"
 #include "libc/log/rop.internal.h"
-#include "libc/mem/gc.internal.h"
+#include "libc/mem/gc.h"
 #include "libc/mem/mem.h"
 #include "libc/runtime/runtime.h"
 #include "libc/runtime/stack.h"