i#1730 xc docs: Enable all docs but op lists for cross-compiling

Enables BUILD_DOCS by default for all architectures.

For option list documentation generation (drcpusim, drcov, drcachesim), when cross-compiling and we cannot executed the generator, instead we place this into the documentation:

The parameters available are described below:

The options list is not currently able to be generated for cross-compiled
builds (https://github.com/DynamoRIO/dynamorio/issues/1730). Please see the
online documentation or run the tool with -help.

This is better than having no documentation at all in our non-x86 packages.

Issue: #1730