arch/x86: rename reset_vector -> _start

In order to align the entry points for the various stages
on x86 to _start one needs to rename the reset_vector symbol.
The section is the same; it's just a symbol change.

Change-Id: I0e6bbf1da04a6e248781a9c222a146725c34268a
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/13881
Tested-by: build bot (Jenkins)
Reviewed-by: Andrey Petrov <andrey.petrov@intel.com>

src/cpu/x86/16bit/reset16.inc

@@ -1,7 +1,7 @@
 	.section ".reset", "ax", %progbits
 	.code16
-.globl	reset_vector
-reset_vector:
+.globl	_start
+_start:
 	.byte  0xe9
 	.int   _start16bit - ( . + 2 )
 	/* Note: The above jump is hand coded to work around bugs in binutils.