CompilerABI not present in recent versions of BinaryBuilder

[fspreck-indiscale]

When trying to reproduce the BinaryBuilder example in testlib-builder with Julia 1.6.1 and BinaryBuilder 0.4.0, the execution fails since CompilerABI() is not defined -- it seems to have been removed from BinaryBuilder. A simpe workaround fro me was to simply replace the generation of the Platforms list by the supported_platforms function from BinaryBuilder, but I'm not sure whether this will produce the desired results for everyone on any platform.