Restore x18 in efi_arch_leave.

Some UEFI implementations trash this register and, as we use it as a
platform register, the kernel doesn't save it before calling into the UEFI
runtime services. As we have a copy in tpidr_el1 restore from there when
exiting the EFI environment.

PR:		237234, 237055
Reviewed by:	manu
Tested On:	Ampere eMAG
MFC after:	2 weeks
Sponsored by:	DARPA, AFRL
Sponsored by:	Ampere Computing (hardware)
Differential Revision:	https://reviews.freebsd.org/D20127

sys/arm64/arm64/efirt_machdep.c

@@ -259,6 +259,16 @@ efi_arch_leave(void)
 {
 	struct thread *td;
 
+	/*
+	 * Restore the pcpu pointer. Some UEFI implementations trash it and
+	 * we don't store it before calling into them. To fix this we need
+	 * to restore it after returning to the kernel context. As reading
+	 * curthread will access x18 we need to restore it before loading
+	 * the thread pointer.
+	 */
+	__asm __volatile(
+	    "mrs x18, tpidr_el1	\n"
+	);
 	td = curthread;
 	__asm __volatile(
 	    "msr ttbr0_el1, %0	\n"