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/runtime/valist.h

@@ -0,0 +1,17 @@
+#ifndef COSMOPOLITAN_LIBC_RUNTIME_VALIST_H_
+#define COSMOPOLITAN_LIBC_RUNTIME_VALIST_H_
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+struct __va {
+  unsigned int gp_offset;
+  unsigned int fp_offset;
+  void *overflow_arg_area;
+  void *reg_save_area;
+};
+
+void *__va_arg(struct __va *, unsigned long, unsigned, unsigned);
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_RUNTIME_VALIST_H_ */