NFC processing time script identifies tests by output filename.
When `/dev/null` is used as output filename, we're unable to tell the
source test, and the reports are unhelpful.
Replace `/dev/null/` with `%t.null` which resolves the issue.
Replace a single dash with a double dash for options that have more
than a single letter.
llvm-bolt-wrapper.py has special treatment for output options such as
"-o" and "-w" causing issues when a single dash is used, e.g. for
"-write-dwp". The wrapper can be fixed as well, but using a double dash
has other advantages as well.
Reviewed By: rafauler
Differential Revision: https://reviews.llvm.org/D127538
