Fix clang warning about redundant link libraries

* Alter meson for Android cross builds.

* Handle liblog linking on Termux as well as the Android NDK.

Make compiler flags for reproducible builds optional

no change in default behavior: -ffile-prefix-map is still added if
supported by compiler but add option for disabling it as it breaks
generic tools for extracting debuginfo/debugsource from build artifacts
like: https://sourceware.org/git/?p=debugedit.git;a=blob;f=scripts/find-debuginfo.in

This removes about 200 pointless copies from printing the help
message (the original motivation for the change), and also nicely
improves performance of the various reporters that depend on
TextFlow.

The code is now even worse mess than before, due to the ad-hoc
implementation of Result-ish type based on virtual functions in
Clara, but it has dropped the allocations for empty binary down
to 151.

Because TokenStream is copied around a lot, moving it to use
`StringRef` removes _a lot_ of allocations per `Opt` in the parser.
Args are not copied around much, but changing them as well makes it
obvious that they do not participate in the ownership.

The changes add up to removing ~180 allocations for "empty"
invocation of the test binary.
(`./tests/ExtraTests/NoTests --allow-running-no-tests -o /dev/null`
is down to 317 allocs)