kbuild: rename built-in.o to built-in.a

Incremental linking is gone, so rename built-in.o to built-in.a, which
is the usual extension for archive files.

This patch does two things, first is a simple search/replace:

git grep -l 'built-in\.o' | xargs sed -i 's/built-in\.o/built-in\.a/g'

The second is to invert nesting of nested text manipulations to avoid
filtering built-in.a out from libs-y2:

-libs-y2 := $(filter-out %.a, $(patsubst %/, %/built-in.a, $(libs-y)))
+libs-y2 := $(patsubst %/, %/built-in.a, $(filter-out %.a, $(libs-y)))

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/namespace.pl

@@ -164,7 +164,7 @@ sub linux_objects
 	s:^\./::;
 	if (/.*\.o$/ &&
 		! (
-		m:/built-in.o$:
+		m:/built-in.a$:
 		|| m:arch/x86/vdso/:
 		|| m:arch/x86/boot/:
 		|| m:arch/ia64/ia32/ia32.o$: