percpu: refactor percpu.h

Impact: cleanup

Refactor the DEFINE_PER_CPU_* macros and add .data.percpu.first
section.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/asm-generic/vmlinux.lds.h

@@ -486,6 +486,7 @@
  */
 #define PERCPU_VADDR(vaddr, phdr)					\
 	PERCPU_PROLOG(vaddr)						\
+		*(.data.percpu.first)					\
 		*(.data.percpu.page_aligned)				\
 		*(.data.percpu)						\
 		*(.data.percpu.shared_aligned)				\