Multiple build failures from new nightlies

[gwilymk]

Nightly releases contain some bugs that mean agb doesn't compile.

• Undefined reference in unchecked_shl function when compiling in release mode for thumbv4t-none-eabi rust-lang/rust#108893
• Cannot represent a difference across sections rust-lang/rust#109730
• build-std seems broken in nightly (rustc 1.71.0-nightly (74c482104 2023-05-04)) rust-lang/rust#111225
• ICE: compiler/rustc_monomorphize/src/collector.rs:1038:9: no MIR available rust-lang/rust#111320

We will keep track of these failures. If you are using a released agb version, instead you can use the nightly version as of the day it was released. If you're using master, we will mitigate issues where possible, check the nightly action build for the most recent working nightly build.

In order to use a different Rust version, you can change the version in the rust-toolchain.toml file to explicitly state the nightly version, for example

[toolchain]
channel = "nightly-2023-05-04"

[gwilymk]

Another issue (different from the first) rust-lang/rust#109730

[corwinkuiper]

Re: the first issue:

• We resolved the issue by switching from opt-level of s to 3 for the release mode build.
• We then had to resolve a run time issue in one of the save tests (potential miscompilation). Switch from lto of "thin" to "fat".