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