Auto merge of #53648 - japaric:thumb-lld, r=alexcrichton

Auto merge of #53648 - japaric:thumb-lld, r=alexcrichton

change the default linker of the ARM Cortex-M targets

to rust-lld so users won't need an external linker to build programs

This will break nightly builds.

We discussed this within the embedded WG and with the embedded community in
rust-embedded/wg#160 and there was consensus in that this breaking change is
worthwhile and that we should do it now before it becomes impossible to do
without breaking stable builds.

We have already written an announcement (see rust-embedded/wg#196) that explains
the breakage and instructs the users how to fix their builds. The TL;DR is that
they can switch to the old behavior by passing the `-C linker` flag to rustc.
We'll post the announcement as soon as this change makes into nightly.

closes rust-embedded/wg#160

r? @alexcrichton