Enable Linux/uClibc-ng toolchain build in CI

[TommyMurphyTM1234]

See:

• See https://github.com/riscv-collab/riscv-gnu-toolchain/issues/1566

Notes:

1. I don't think that I can test these changes locally so am creating the PR on the assumption that they will be sanity checked server side.
2. While "ulibc" is used across riscv-gnu-toolchain to refer to this toolchain variant, it is actually uClibc-ng based and not based on the original uClibc.
3. For this reason I have used "uclibc-ng" when naming the build tarball artifact but I don't know if the dash in this string will be in any way problematic or inconsistent with the existing naming convention?
4. I may need to rebase this if/when other pending PRs that I have submitted today are merged.
5. I am assuming that, like the Linux/musl toolchain variant. LLVM is also not an option for the uClibc-ng toolchain?
6. As ever, I'm open to constructive feedback and comments. :-)

[TommyMurphyTM1234]

Second attempt at doing this:

• https://github.com/riscv-collab/riscv-gnu-toolchain/pull/1574

[cmuellner]

It looks good now ("This branch has no conflicts with the base branch when rebasing"). I'll merge once the CI build is done.

[TommyMurphyTM1234]

It looks good now ("This branch has no conflicts with the base branch when rebasing"). I'll merge once the CI build is done.

Thanks @cmuellner. 👍

[TommyMurphyTM1234]

Oops - mistake in original commit addressed here:

• https://github.com/riscv-collab/riscv-gnu-toolchain/pull/1578/commits/10ecf9323b086b1ea4e682455485594e878f366d

[TommyMurphyTM1234]

Thanks a lot @cmuellner. 👍