amd64: Use a larger boot stack

amd64: Use a larger boot stack

With sanitizers enabled, it becomes possible to overflow the stack when
only a single page is used.  Follow arm64's example and use the default
kernel stack size instead.  This is a bit wasteful, but without a guard
page, overflow merely corrupts adjacent .bss entries and is thus
difficult to debug.

Note, with a GENERIC kernel we already consume over half of the
available boot stack space, see the review for an example.

Reviewed by:	kib
Reported by:	Jenkins
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D41166