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>
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 64155e3..5be22e4 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -324,11 +324,11 @@
 	    the analysis would not catch the illegal reference.
 	    This option tells gcc to inline less (but it does result in
 	    a larger kernel).
-	  - Run the section mismatch analysis for each module/built-in.o file.
+	  - Run the section mismatch analysis for each module/built-in.a file.
 	    When we run the section mismatch analysis on vmlinux.o, we
 	    lose valuable information about where the mismatch was
 	    introduced.
-	    Running the analysis for each module/built-in.o file
+	    Running the analysis for each module/built-in.a file
 	    tells where the mismatch happens much closer to the
 	    source. The drawback is that the same mismatch is
 	    reported at least twice.