Fix coredump_phnum test with ASLR enabled by default

Fix coredump_phnum test with ASLR enabled by default

coredump_phnum intends to generate a core file with many PT_LOAD
segments.  Previously it called mmap() in a loop with alternating
protections, relying on each mapping following the previous, to produce
a core file with many page-sized PT_LOAD segments.  With ASLR on we no
longer have this property of each mmap() following the previous.

Instead, perform a single allocation, and then use mprotect() to set
alternating pages to PROT_READ.

PR:		259970
Reported by:	lwhsu, mw
Reviewed by:	kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D33070