Expose public garbage collector API for C language

You can now do epic things like this:

    puts(_gc(xasprintf("%d", 123)));

The _gc() API is shorthand for _defer() which works like Go's keyword:

    const char *s = xasprintf("%d", 123);
    _defer(free, s);
    puts(s);

Be sure to always use -fno-omit-frame-pointer which makes code fast too.

Enjoy! See also #114

libc/nexgen32e/gclongjmp.S

@@ -29,7 +29,7 @@
 //	@assume	system five nexgen32e abi conformant
 //	@see	examples/ctrlc.c
 //	@noreturn
-gclongjmp:
+_gclongjmp:
 	.leafprologue
 	.profilable
 	.weak	__garbage
@@ -57,5 +57,4 @@ gclongjmp:
 2:	pop	%rsi
 	pop	%rdi
 	jmp	0b
-	.endfn	gclongjmp,globl
-	.source	__FILE__
+	.endfn	_gclongjmp,globl