[libcxx][CI] Use lld for everything in the ARM picolib builds (#158320)

Our host compiler is a clang install that will default to ld if not told
otherwise.

We were telling meson to use lld, but the way that we did it was
outdated, which lead to picolib producing a linker script that lld could
not use. The tests were in fact linking with ld instead.

Using the `c_ld` setting fixes this problem. See:
https://mesonbuild.com/Machine-files.html#binaries

Then to use lld in tests we need `-fuse-ld=lld` in the config files.

Some of these options were not needed for clang 19.1.7, but were for
clang 21.1.1. We will soon update to 21.1.1 so I have included all of
the required options in this PR.

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