kbuild: move vmlinux.a build rule to scripts/Makefile.vmlinux_a

kbuild: move vmlinux.a build rule to scripts/Makefile.vmlinux_a

Move the build rule for vmlinux.a to a separate file in preparation
for supporting distributed builds with Clang ThinLTO.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Rong Xu <xur@google.com>
Tested-by: Piotr Gorski <piotrgorski@cachyos.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Rong Xu <xur@google.com>
Link: https://patch.msgid.link/20260529185347.2418373-2-xur@google.com
[nathan: Squash in forward fix from Rong around '--thin' to $(AR)
         https://patch.msgid.link/20260529185347.2418373-3-xur@google.com]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>