Fix build-std settings for compiler-builtins

This is a workaround to ensure intrinsics get linked in from avr-libc
instead of Rust's compiler-builtins.  The problem is that the versions
from compiler-builtins have slightly different calling convention and as
such cannot actually be substituted for the versions from avr-libc right
now.

This should fix a lot of the observed problems with 32-bit arithmetic.

Ref: rust-lang/rust#82242 (comment)

.cargo/config.toml

@@ -32,6 +32,7 @@
 
 [unstable]
 build-std = ["core"]
+build-std-features = ["compiler-builtins-mangled-names"]
 {%- comment %}
 # vim: ft=toml.jinja2
 {% endcomment %}