Add thumbv7m/thumbv7em targets to CI

[tec-eli]

Description

CI currently only cross-compiles for thumbv6m-none-eabi (Cortex-M0). We need coverage for
Cortex-M3/M4/M7 targets before calling the no_std story validated.

• Scope: .github/workflows/ci.yml

Tasks

• Add thumbv7m-none-eabi to the toolchain install step
• Add thumbv7em-none-eabihf to the toolchain install step
• Add build-only jobs for both new targets (cargo build --no-default-features --target ...)
• Verify none of the new targets link against std