Fix schema generation on alpine

Alpine uses musl libc, which is identified as a different compilation
target by the Rust compiler. This target was not configured to use pgx's
magical linker script, so the symbols which the schema generator expects
to be present were not present.

Fixes #340

cargo-pgx/src/templates/cargo_config

@@ -6,6 +6,9 @@ linker = ".cargo/pgx-linker-script.sh"
 [target.aarch64-unknown-linux-gnu]
 linker = ".cargo/pgx-linker-script.sh"
 
+[target.aarch64-unknown-linux-musl]
+linker = ".cargo/pgx-linker-script.sh"
+
 [target.x86_64-apple-darwin]
 linker = ".cargo/pgx-linker-script.sh"