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>

usr/initramfs_data.S

@@ -10,7 +10,7 @@
 
   ld -m elf_i386  --format binary --oformat elf32-i386 -r \
   -T initramfs_data.scr initramfs_data.cpio.gz -o initramfs_data.o
-   ld -m elf_i386  -r -o built-in.o initramfs_data.o
+   ld -m elf_i386  -r -o built-in.a initramfs_data.o
 
   For including the .init.ramfs sections, see include/asm-generic/vmlinux.lds.