* Create mirror for system with different compilers
Spack concretizes the spec provided by the user in
"spack mirror create" to ensure downloading the right
dependencies. Under normal circumstances concretization
requires that the chosen compiler exists on the system,
but this is not required when creating download mirrors
for other systems, so this requirement is removed in that
case.
* Add test for disabling compiler existence check
* Update compiler existence checking logic
* improve test for disabling compiler existence check
6548f2db5c