external_deps: properly build native tools, make possible to loop some builds for different targets, hardlink deduplication before taring…

[illwieckz]

Merging more stuff from:

• Rework external_deps for Saigo and CMake-based NaCl rebuild #1986

First rounds were:

• Tidying on external_deps #1992
• external_deps: add download action to download and extract the external deps tarball like CMake #1985

This doesn't bump the deps version.

• Deduplicate files with hardlinks before taring the Windows tarball, similar to what we now do with the Saigo NaCl SDK. It's the best way to guarantee deduplication in tarball, while it's meant to be properly expanded on Linux or Windows (at worse, hardlinks are turned into copies on Windows). The deduplication can use either jdupes or rdfind. We can settle on jdupes if that's preferred. If none of those tools are present files are stored as normal files. Symlink dereference is still done but now done before the hardlink creation.
• Make possible to reset the defaults (compilers, flags), and reconfigure a build for a different target. It makes things more robust in general, and make possible to loop some build. It makes possible for example to build NaCl loaders for both linux-amd64 and linux-armhf when building deps for linux-arm64 (not implemented in that PR). This may also make possible to build the same software as a native tool and to rebuild it for the target if needed.
• Add more reusable variable describing the current build (preparatory work).
• Deduplicate some NaCl loader copy code from PNaCl.
• Make possible to build tools as native tools the proper way, introducing the native platform and building native tools in a special prefix that doesn't contribute to the target package.
• Build pkg-config and nasm as native tools.