Fix linker error: undefined symbol core::panicking::panic

Builds with `-Z build-std-features=panic_immediate_abort` to ensure the
`core::panicking` plumbing is stripped.

See: rust-lang/rust#55011

Adds `-Cpanic=abort` to RUSTFLAGS so that build profiles do not need to
specify `panic = "abort"` in Cargo.toml. This can potentially allow
simulator builds to customize panics. (Maybe printing to the console and
stopping the event loop, for instance. But this work is TBD.)

Fixes pd-rs/crankstart#66

src/main.rs

@@ -651,6 +651,7 @@ impl Build {
             args.push("thumbv7em-none-eabihf");
 
             args.push("-Zbuild-std=core,alloc");
+            args.push("-Zbuild-std-features=panic_immediate_abort");
         }
 
         let envs = if self.device {
@@ -661,6 +662,7 @@ impl Build {
                     "-Ctarget-cpu=cortex-m7",
                     "-Clink-args=--emit-relocs",
                     "-Crelocation-model=pic",
+                    "-Cpanic=abort",
                 ]
                 .join(" "),
             );