Rewrite chip startup code in assembly

[MabezDev]

See rust-embedded/cortex-m-rt#300 for me details.

The TLDR version is, we shouldn't be calling any Rust until data/bss and properly initialized. asm! is stable so we should just do it :)

[bjoernQ]

Relevant issues

• Replace use of the r0 crate with inline assembly #1816
• Replace use of the r0 crate with inline assembly esp-riscv-rt#1

[jessebraham]

There's a tracking issue in xtensa-lx already for this, so going to close this issue since there's nothing left to do once that's updated.