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.

build/definitions.mk

@@ -146,8 +146,6 @@ DEFAULT_CFLAGS =							\
 	-std=gnu2x
 
 DEFAULT_CXXFLAGS =							\
-	-fno-rtti							\
-	-fno-exceptions							\
 	-fuse-cxa-atexit						\
 	-Wno-int-in-bool-context					\
 	-Wno-narrowing							\