In commit v3.6.0-rc1~88^2 (CustomCommandGenerator: Add support for
CROSSCOMPILING_EMULATOR, 2016-05-04) logic was introduced to substitute
a target's `CROSSCOMPILING_EMULATOR` for argv0 in a custom command.
However, it broke the case when the argv0 was a target name and now
fails to expand the target name to its location at the same time as
inserting the emulator.  Fix the latter case.

Inspired-by: Brian Maher <brian@brimworks.com>
Closes: #16288