[GH #268] sysctl: check for HW_MEMSIZE

HW_PHYSMEM returns max 2G as its a signed int only. Use HW_MEMSIZE if available.
This helps rakudo building core.pir doing less gc, which is triggered when 10% of total memory is allocated
Merge remote-tracking branch 'origin/rurban/darwin-memsize-gh268'
[configure] quote libdir path for inst_libparrot_linkflags

otherwise pbc_to_exe does not work with spacey paths when installed
[doc] Update changelog
[doc][ci skip] Fix typo
Prepare for the 5.1.0 release
Add "Build" category to the changelog skeleton
Update version numbers for the 5.1.0 release
Update info for this release and next release
Merge branch 'master' into eval_pmc