Add documentation for Cortex-M23/M33/M35P

[NZSmartie]

This is sort of a cross-posted issue from rust-embedded/cortex-m-quickstart#94

I'm getting started with a ATSAML10xxxx series project that uses Cortex M23 (ARM v8 baseline)

Since I'm new to Rust development (but already an experienced embedded developer), I've been following The Embedded Rust Book - Installing the tools. Specifically it suggests adding the following targets with rustup target add

• thumbv6m-none-eabi
• thumbv7m-none-eabi
• thumbv7em-none-eabi
• thumbv7em-none-eabihf

As for ARM v8 baseline, mainline and mainline with extension, the targets are respectively:

• thumbv8m.base-none-eabi (e.g. Cortex M23)
• thumbv8m.main-none-eabi (Not sure which arch uses this instruction set specifically 🤷‍♀️)
• thumbv8m.main-none-eabihf (e.g. Cortex M33, CortexM35P)

Could these be added to src/intro/install.md and also other places I've yet to come across?

book/src/intro/install.md

Lines 14 to 40 in 0cd2ca1

	rustc 1.31.1 (b6c32da9b 2018-12-18)
	```
	For bandwidth and disk usage concerns the default installation only supports
	native compilation. To add cross compilation support for the ARM Cortex-M
	architectures choose one of the following compilation targets. For the STM32F3DISCOVERY
	board used for the examples in this book, use the final `thumbv7em-none-eabihf` target.
	Cortex-M0, M0+, and M1 (ARMv6-M architecture):
	``` console
	$ rustup target add thumbv6m-none-eabi
	```
	Cortex-M3 (ARMv7-M architecture):
	``` console
	$ rustup target add thumbv7m-none-eabi
	```
	Cortex-M4 and M7 without hardware floating point (ARMv7E-M architecture):
	``` console
	$ rustup target add thumbv7em-none-eabi
	```
	Cortex-M4F and M7F with hardware floating point (ARMv7E-M architecture):
	``` console
	$ rustup target add thumbv7em-none-eabihf
	```

Maybe a reference to the Cortex-M comparison table might help with picking out the right instruction set

[eldruin]

Sounds fine to me, although that list is getting longer. Could you send a PR to this repo adding these?