Document --print-supported-extensions and --print-enabled-extensions

[lenary]

This documentation is based on the implementation in Clang.

[cmuellner]

Is there a flag to list all supported mcpu identifiers?

[lenary]

Is there a flag to list all supported mcpu identifiers?

Clang treats -mcpu=help and -mtune=help specially - and on clang both of these have identical output (but might not if -mtune= becomes more complex :) )

[lenary]

IIRC, the -mcpu=help lists take into account 32- vs 64-bit CPUs (depending on the triple given to clang). I don't think that --print-supported-extensions does so, it prints everything (this difference is less relevant to --print-enabled-extensions).