x86_64: fix .size of _init/_fini

binutils-2.21 barf on .size that do not evaluate to const, so use the
section size and not a function that is not visible here.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libc/sysdeps/linux/x86_64/crtn.S

@@ -7,7 +7,7 @@
 .type    _init, %function
 	addq $8, %rsp
 	ret
-.size _init,.-_init
+.size _init,.-.init
 
 
 .section .fini
@@ -15,4 +15,4 @@
 .type    _fini, %function
 	addq $8, %rsp
 	ret
-.size _fini, .-_fini
+.size _fini, .-.fini