Update compile_to_ppc build instructions

Author: aprilwade

compile_to_ppc/.cargo/config

@@ -0,0 +1,5 @@
+[build]
+target-dir = "../target"
+
+[target.powerpc-unknown-linux-gnu]
+rustflags = ["-C", "relocation-model=static", "-C", "target-cpu=750"]

compile_to_ppc/README.md

@@ -1,12 +1,11 @@
-This directory contains crates that are compiled to PPC and some associated helper proc-macros. Compiling these crates requires having the nightly compiler and the `powerpc-unknown-linux-gnu` target installed. You can install them with `rustup`:
+This directory contains crates that are compiled to PPC and some associated helper proc-macros. Compiling these crates requires having  the `powerpc-unknown-linux-gnu` target installed. You can install them with `rustup`:
 
 ```
-rustup toolchain install nightly
-rustup target add --toolchain nightly powerpc-unknown-linux-gnu
+rustup target add --toolchain stable powerpc-unknown-linux-gnu
 ```
 
-The recommended command to compile any of these crates is:
+A nightly-only feature is required to build `primeapi-rs`, so the `RUSTC_BOOTSTRAP=1` environment variable must be set when building most anything in this directory. Thus, the recommended command to compile any of these crates is:
 
 ```
-cargo +nightly rustc --release --target powerpc-unknown-linux-gnu -- -C relocation-model=static
+RUSTC_BOOTSTRAP=1 cargo rustc -p rel_patches --release --target powerpc-unknown-linux-gnu -- -C relocation-model=static
 ```