ARM: vmlinux.lds: use _text and _stext the same way as x86

x86 uses _text to mark the start of the kernel image including the
head text, and _stext to mark the start of the .text section.  Change
our vmlinux.lds to conform.  An audit of the places which use _stext
and _text in arch/arm indicates no users of either symbol are impacted
by this change.  It does mean a slight change to /proc/iomem output.
Acked-by: NNicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/vmlinux.lds.S

@@ -66,11 +66,11 @@ SECTIONS
 	. = PAGE_OFFSET + TEXT_OFFSET;
 #endif
 	.head.text : {
-		_stext = .;
+		_text = .;
 		HEAD_TEXT
 	}
 	.text : {			/* Real text segment		*/
-		_text = .;		/* Text and read-only data	*/
+		_stext = .;		/* Text and read-only data	*/
 			__exception_text_start = .;
 			*(.exception.text)
 			__exception_text_end = .;