Properly mark mmap_ent as global

It currently happens to work fine using cg_llvm as the global_asm!()
ends up in the same cgu as the use of this global. There is no guarantee
that this will always be the case and in fact in case of cg_clif this
happens to not be the case, thus causing a linker error.

Author: bjorn3

Diff for: src/e820.s

@@ -51,4 +51,5 @@ do_e820:
 	stc			# "function unsupported" error exit
 	ret
 
+.global mmap_ent
 mmap_ent: .word 0