vm: Simplify startup page dumping conditional

This commit introduces the MINIDUMP_STARTUP_PAGE_TRACKING symbol and
uses it to simplify several instances of a complex preprocessor conditional
for adding pages allocated when bootstraping the kernel to minidumps.

Reviewed by: markj, mhorne
Approved by: markj (mentor)
Differential Revision: https://reviews.freebsd.org/D45085