Fix custom command target substitution with CROSSCOMPILING_EMULATOR
In commit 8c2cedc6 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.
Closes: #16288 (closed)