toolchain: arm: Add Zephyr defines and C/C++ standard features to cortex-m_llvm

The cortex-m_llvm cc_toolchain was missing zephyr_mode_defines in its
args (such as -D__ZEPHYR__=1 and -DKERNEL) and was missing the C/C++
standard features in enabled_features and known_features.

Add //toolchain/args:zephyr_mode_defines and the C/C++ standard feature
selects so that cortex-m_llvm targets properly compile with the
configured C and C++ standards (e.g. C++20) and standard Zephyr defines.
Place feature selects before :llvm_cortex_libc to maintain the required
feature ordering invariant.

Change-Id: Ib1b84f2ac509325e914fa0010d279e37915f771a
Reviewed-on: https://pigweed-review.googlesource.com/c/third_party/github/zephyrproject-rtos/zephyr/+/469645