Evaluate generator expressions in these properties, as they apply to
`add_test`, `add_custom_command`, and `add_custom_target`.
The `CMAKE_CROSSCOMPILING_EMULATOR` variable's `try_run` behavior occurs
at configure time and so cannot support generator expressions.
In commit fec441ec17 (Teach CROSSCOMPILING_EMULATOR to support
arguments, 2019-05-30, v3.15.0-rc1~6^2) the new member
`cmCustomCommandGenerator::EmulatorsWithArguments` was not initialized
to the proper size. Fix this and add a test case covering the crash
that could occur with multiple commands where an emulator appears only
in a later command.
Fixes: #19500
Co-Author: Brad King <brad.king@kitware.com>
Teach the `add_custom_command` and `add_custom_target' commands to
substitute argv0 with the crosscompiling emulator if it is a target with
the `CROSSCOMPILING_EMULATOR` property set.
