[LLVM][Release] Prevent empty runtime name in release script

Unlike projects, runtimes doesn't have a default set of names.
This means you get a leading space at the start, which gets converted
to a ';' giving ";<runtime name>;<runtime name>".

CMake then errors because the "" before the first ';' is treated
as a runtime name and of course it's not a valid name.

Fix this by removing the leading spaces from runtimes before we
insert the ';'.

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D139306