x86, boot: set up the decompression stack as early as possible

Set up the decompression stack as soon as we know where it needs to
go.  That way we have a full-service stack as soon as possible, rather
than relying on the BP_scratch field.

Note that the stack does need to be empty during bss zeroing (or
else the stack needs to be moved out of the bss segment, which is also
an option.)

[ Impact: cleanup, minor paranoia ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>