rust: Introduce RUST_BUILD_ARCH

RUST_BUILD_ARCH contains the arch_to_rust_arch()-translated value of
BUILD_ARCH. This is necessary to acquire the correct snapshot artifacts
under Linux where `uname -m` reports "ppc64le" rather than
"powerpc64le".

(cherry picked from commit a6be6aa40f81cffe155ea7dafc891ee8db615455)
Change-Id: I6aec23aced8e1c6f0bfc46fe52531b0c16bcf687
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
diff --git a/poky/meta/recipes-devtools/rust/rust-snapshot.inc b/poky/meta/recipes-devtools/rust/rust-snapshot.inc
index 74b5582..173648c 100644
--- a/poky/meta/recipes-devtools/rust/rust-snapshot.inc
+++ b/poky/meta/recipes-devtools/rust/rust-snapshot.inc
@@ -15,11 +15,11 @@
 SRC_URI[cargo-snapshot-aarch64.sha256sum] = "6d11cd94618d80cda273eeeae7285980445f61a49ebacc616777b482a41cbf3f"
 
 SRC_URI += " \
-    https://static.rust-lang.org/dist/${RUST_STD_SNAPSHOT}.tar.xz;name=rust-std-snapshot-${BUILD_ARCH};subdir=rust-snapshot-components \
-    https://static.rust-lang.org/dist/${RUSTC_SNAPSHOT}.tar.xz;name=rustc-snapshot-${BUILD_ARCH};subdir=rust-snapshot-components \
-    https://static.rust-lang.org/dist/${CARGO_SNAPSHOT}.tar.xz;name=cargo-snapshot-${BUILD_ARCH};subdir=rust-snapshot-components \
+    https://static.rust-lang.org/dist/${RUST_STD_SNAPSHOT}.tar.xz;name=rust-std-snapshot-${RUST_BUILD_ARCH};subdir=rust-snapshot-components \
+    https://static.rust-lang.org/dist/${RUSTC_SNAPSHOT}.tar.xz;name=rustc-snapshot-${RUST_BUILD_ARCH};subdir=rust-snapshot-components \
+    https://static.rust-lang.org/dist/${CARGO_SNAPSHOT}.tar.xz;name=cargo-snapshot-${RUST_BUILD_ARCH};subdir=rust-snapshot-components \
 "
 
-RUST_STD_SNAPSHOT = "rust-std-${RS_VERSION}-${BUILD_ARCH}-unknown-linux-gnu"
-RUSTC_SNAPSHOT = "rustc-${RS_VERSION}-${BUILD_ARCH}-unknown-linux-gnu"
-CARGO_SNAPSHOT = "cargo-${CARGO_VERSION}-${BUILD_ARCH}-unknown-linux-gnu"
+RUST_STD_SNAPSHOT = "rust-std-${RS_VERSION}-${RUST_BUILD_ARCH}-unknown-linux-gnu"
+RUSTC_SNAPSHOT = "rustc-${RS_VERSION}-${RUST_BUILD_ARCH}-unknown-linux-gnu"
+CARGO_SNAPSHOT = "cargo-${CARGO_VERSION}-${RUST_BUILD_ARCH}-unknown-linux-gnu"