Clang 20.1.0

[kilograham]

It seems that

• LLVM Embedded Toollchain for Arm has become Arm Toolchain for Embedded
• For unaligned-access safe targets, the defaut/only runtime now specifies _unaligned in the name

This PR adds the armv8m runtime _unaligned variant to our whitelist of runtimes to look for

fixes #2534

[kilograham]

@will-v-pi do you wanna add it to our matrix?

[will-v-pi]

@will-v-pi do you wanna add it to our matrix?

Is this the multi-gcc.yml workflow? I think adding to that requires @liamfraser to also add the new compiler to /opt/arm on the test system.