Clean up some code

- Use good ELF technique in cosmo_dlopen()
- Make strerror() conform more to other libc impls
- Introduce __clear_cache() and use it in cosmo_dlopen()
- Remove libc/fmt/fmt.h header (trying to kill off LIBC_FMT)

libc/nt/kernel32/FormatMessageA.S

@@ -0,0 +1,18 @@
+#include "libc/nt/codegen.h"
+.imp	kernel32,__imp_FormatMessageA,FormatMessageA
+
+	.text.windows
+        .ftrace1
+FormatMessageA:
+        .ftrace2
+#ifdef __x86_64__
+	push	%rbp
+	mov	%rsp,%rbp
+	mov	__imp_FormatMessageA(%rip),%rax
+	jmp	__sysv2nt8
+#elif defined(__aarch64__)
+	mov	x0,#0
+	ret
+#endif
+	.endfn	FormatMessageA,globl
+	.previous