amd64: Make it easier to configure exception stack sizes

The amd64 kernel handles certain types of exceptions on a dedicated
stack. Currently the sizes of these stacks are all hard-coded to
PAGE_SIZE, but for at least NMI handling it can be useful to use larger
stacks. Add constants to intr_machdep.h to make this easier to tweak.

No functional change intended.

Reviewed by: kib
MFC after: 1 week
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27076