build: Support building target in (thumbv7em-none-eabi, thumbv7em-none-eabihf) based on stm32h7xx.

```toml
[build]
target = "thumbv7em-none-eabi"       # Cortex-M4 and Cortex-M7 (no FPU)
target = "thumbv7em-none-eabihf"     # Cortex-M4F and Cortex-M7F (with FPU)
```

which also requires the upstream crate rust-amplify for building the target in (`thumbv7em-none-eabi`, `thumbv7em-none-eabihf`) based on stm32h7xx.