Building head on stable/11 requires libzstd

Add lib/libzstd to _elftoolchain_libs

tools/build/Makefile needs to create the install dir for libzstd
Since this would make the line too long, rework to use a list
in one per line format (easier to add in future)
and dispense with the .for loop

Reviewed by: emaste bapt
Differential Revision: https://reviews.freebsd.org/D220134