x86/boot/compressed/64: Use LEA to initialize boot stack pointer

It's shorter, and it's what is used in every other place, so make it
consistent.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200107194436.2166846-2-nivedita@alum.mit.edu