Get codebase completely working with LLVM

You can now build Cosmopolitan with Clang:

    make -j8 MODE=llvm
    o/llvm/examples/hello.com

The assembler and linker code is now friendly to LLVM too.
So it's not needed to configure Clang to use binutils under
the hood. If you love LLVM then you can now use pure LLVM.

libc/runtime/exit.S

@@ -20,10 +20,10 @@
 .text.exit
 .source	__FILE__
 
-/	Exits program with grace.
-/
-/	@param	%dil has exit code
-/	@noreturn
+//	Exits program with grace.
+//
+//	@param	%dil has exit code
+//	@noreturn
 exit:	push	%rbp
 	mov	%rsp,%rbp
 	push	%rdi