Add FreeBSD case to gcenv.

[Thefrank]

Closes #64935

[ghost]

Tagging subscribers to this area: @dotnet/gc
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Closes #64935

Author:	Thefrank
Assignees:	-
Labels:	area-GC-coreclr, community-contribution
Milestone:	-

[Thefrank]

Calculated based on #64935 (comment) and reworked from #64935 (comment). I used vm.stats.vm.v_laundry_count as cache always returned 0 for me

sysctl -d vm.stats.vm.v_inactive_count vm.stats.vm.v_free_count vm.stats.vm.v_laundry_count vm.stats.vm.v_cache_count
vm.stats.vm.v_inactive_count: Inactive pages
vm.stats.vm.v_free_count: Free pages
vm.stats.vm.v_laundry_count: Pages eligible for laundering
vm.stats.vm.v_cache_count: Dummy for compatibility

sysctl -h vm.stats.vm.v_inactive_count vm.stats.vm.v_free_count vm.stats.vm.v_laundry_count vm.stats.vm.v_cache_count
vm.stats.vm.v_inactive_count: 8,474,860
vm.stats.vm.v_free_count: 14,076,453
vm.stats.vm.v_laundry_count: 48,202
vm.stats.vm.v_cache_count: 0

Notes for future users:

• sysconf on FreeBSD is basically sysctl https://github.com/freebsd/freebsd-src/blob/e04c4b4a369df3f1dcbebbdf726193f02af60801/lib/libc/gen/sysconf.c however sysconf is generally more portable than sysctl
• I found that the code was mostly duplicated between src/coreclr/gc/unix/gcenv.unix.cpp and src/coreclr/pal/src/misc/sysinfo.cpp and added FreeBSD to both
• Reverts disabled test from disable GetGCMemoryInfo on FreeBSD #64937

[am11]

GlobalMemoryStatusEx in PAL is unused (outside of PAL tests); good candidate of deletion?
i.e. everything this query reports: git grep GlobalMemoryStatusEx ':/src/coreclr/pal'

[jkotas]

I have submitted the cleanup in separate PR: #91554

@Thefrank Could you please revert changes in this file? It is dead code.

[jkotas]

LGTM. Thank you!

[Thefrank]

Updated title to reflect changed files.

@jkotas @am11 Thank you both for your continued assistance with community platforms!