Add chibicc

This program popped up on Hacker News recently. It's the only modern
compiler I've ever seen that doesn't have dependencies and is easily
modified. So I added all of the missing GNU extensions I like to use
which means it might be possible soon to build on non-Linux and have
third party not vendor gcc binaries.

libc/conv/conv.h

@@ -97,7 +97,7 @@ double RoundDecimalPlaces(double, double, double (*)(double));
 #endif
 
 #ifndef __STRICT_ANSI__
-intmax_t __imaxabs(intmax_t) asm("imaxabs") libcesque pureconst;
+intmax_t __imaxabs(intmax_t) libcesque pureconst;
 #define imaxabs(x) __imaxabs(x)
 #endif /* !ANSI */