Update Module::CoreList for 5.21.2
Listed updated modules.
Acknowledgement section of perldelta
Add new release to perlhist
Adjust 31e62fa1f35be3aceacc7fa62ce72d7131299a9a

It seems that 'sqrt' is not always called as a system call, in particular,
when compiling with -Dusemorebits
Merge branch 'release-5.21.2' into blead

Conflicts:
	t/porting/libperl.t