Add UEFI support

This is mutually exclusive with Windows support. Documentation for how
to use it has been written in libc/runtime/efimain.c

libc/calls/efi.h

@@ -0,0 +1,12 @@
+#ifndef COSMOPOLITAN_LIBC_CALLS_EFI_H_
+#define COSMOPOLITAN_LIBC_CALLS_EFI_H_
+#include "libc/nt/efi.h"
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+extern EFI_HANDLE __efi_image_handle;
+extern EFI_SYSTEM_TABLE *__efi_system_table;
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_CALLS_EFI_H_ */