x86/kexec: Use RIP relative addressing

Normally identity_mapped is not visible to objtool, due to:

  arch/x86/kernel/Makefile:OBJECT_FILES_NON_STANDARD_relocate_kernel_$(BITS).o := y

However, when we want to run objtool on vmlinux.o there is no hiding
it:

  vmlinux.o: warning: objtool: .text+0x4c0f1: unsupported intra-function call

Replace the (i386 inspired) pattern:

	call 1f
  1:	popq %r8
	subq $(1b - relocate_kernel), %r8

With a x86_64 RIP-relative LEA:

	leaq relocate_kernel(%rip), %r8
Suggested-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200324160924.143334345@infradead.org

arch/x86/kernel/relocate_kernel_64.S

@@ -196,10 +196,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 
 	/* get the re-entry point of the peer system */
 	movq	0(%rsp), %rbp
-	call	1f
-1:
-	popq	%r8
-	subq	$(1b - relocate_kernel), %r8
+	leaq	relocate_kernel(%rip), %r8
 	movq	CP_PA_SWAP_PAGE(%r8), %r10
 	movq	CP_PA_BACKUP_PAGES_MAP(%r8), %rdi
 	movq	CP_PA_TABLE_PAGE(%r8), %rax