amd64: Make it easier to configure exception stack sizes

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