kbuild: fix the warning when running make tags

make tags was giving the below warning.

ctags: Warning: arch/x86_64/kernel/head.S:124: null expansion of name
pattern "\1"

Fix the same by making sure we taken only ENTRY pattern found at the
begining of the line.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1333,7 +1333,7 @@ define xtags
 		-I __initdata,__exitdata,__acquires,__releases \
 		-I EXPORT_SYMBOL,EXPORT_SYMBOL_GPL \
 		--extra=+f --c-kinds=+px \
-		--regex-asm='/ENTRY\(([^)]*)\).*/\1/'; \
+		--regex-asm='/^ENTRY\(([^)]*)\).*/\1/'; \
 	    $(all-kconfigs) | xargs $1 -a \
 		--langdef=kconfig \
 		--language-force=kconfig \