[bazel] Factor out rom_start.S into a cc_library
[bazel] Factor out rom_start.S into a cc_library

Signed-off-by: Dan McArdle <8fb240f11ec935676fcfd8aed04f42f9ee63efa8@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan