Rollup merge of rust-lang#56954 - hug-dev:armv8m-main-ci, r=alexcrichton

Add dist builder for Armv8-M Mainline

This commit adds the Armv8-M Mainline target in the list of targets that
get their dist components built. It also update the build-manifest so
that this target gets also its dist components uploaded.

I took example on other pull requests doing the same thing for another target to make the changes. Please feel free to comment if things needs to be added or removed.

Doing `./x.py dist --target thumbv8m.main-none-eabi` worked locally so I assume that this will also work on the CI.
It will (I think) however need a new release of alexcrichton/cc-rs to include the pull request rust-lang/cc-rs#363 @alexcrichton

I hope to do the HardFloat version (`thumbv8m.main-none-eabihf`) and Baseline (`thumbv8m.base-none-eabi`) later, as fixes need to be done on compiler-builtins first to support those.

src/ci/docker/dist-various-1/Dockerfile

@@ -103,6 +103,7 @@ ENV TARGETS=$TARGETS,thumbv6m-none-eabi
 ENV TARGETS=$TARGETS,thumbv7m-none-eabi
 ENV TARGETS=$TARGETS,thumbv7em-none-eabi
 ENV TARGETS=$TARGETS,thumbv7em-none-eabihf
+ENV TARGETS=$TARGETS,thumbv8m.main-none-eabi
 ENV TARGETS=$TARGETS,riscv32imc-unknown-none-elf
 ENV TARGETS=$TARGETS,riscv32imac-unknown-none-elf
 ENV TARGETS=$TARGETS,armebv7r-none-eabi

src/tools/build-manifest/src/main.rs

@@ -100,6 +100,7 @@ static TARGETS: &'static [&'static str] = &[
     "thumbv7em-none-eabi",
     "thumbv7em-none-eabihf",
     "thumbv7m-none-eabi",
+    "thumbv8m.main-none-eabi",
     "wasm32-unknown-emscripten",
     "wasm32-unknown-unknown",
     "x86_64-apple-darwin",