arch/x86: move SetCodeSelector to .text segment

It ended up in .data, and that doesn't seem to be actually necessary.

Change-Id: Ib17d6f9870379d1b7ad7bbd3f16a0839b28f72c8
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/13134
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>

src/arch/x86/c_start.S

@@ -398,6 +398,7 @@ _idt:
 	.fill	20, 8, 0	# idt is uninitialized
 _idt_end:
 
+	.section ".text._start", "ax", @progbits
 #ifdef __x86_64__
 SetCodeSelector:
 	# save rsp because iret will align it to a 16 byte boundary