[BOLT] Fix many tests detected as unsupported

Since D148847, many tests are detected as being unsupported. This is
caused by BOLT_TARGETS_TO_BUILD being ;-separated whereas the previously
used TARGETS_TO_BUILD is space-separated.

This patch fixes this by creating config.targets lit.cfg.py by splitting
on ';'.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D149026