test-release.sh: build projects and runtimes lists with semicolons

While doing a test-release.sh run on FreeBSD, I ran into a sed error due
to the introduction of the GNU extension '\s' in commit 500587e2.

Scanning for blanks (spaces and tabs) could be done in a more portable
fashion using the [[:blank:]] character class. But it is easier to avoid
the original problem, which is that the projects and runtime lists have
to be separated by semicolons, and cannot start with a semicolon.

Instead, use the shell's alternate value parameter expansion mechanism,
which makes it easy to append items to lists with separators in between,
and without any leading separator. This also avoids having to run sed on
the end result.

In addition, build any selected runtimes in the second phase, otherwise
the third phase can fail to find several symbols in compiler-rt, if that
has been built. This is because the host's compiler-rt is not guaranteed
to have those symbols.

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D145884