tc-rust.mk: Fix Rust target detection by moving checks to runtime (#6919)

Rust toolchain and target availability were previously evaluated using
Makefile conditionals (ifeq), which are resolved at parse time and do not
reflect runtime changes made by rustup commands.

This caused incorrect behavior for non-standard Rust toolchains (e.g. qoriq),
where the default rustup toolchain does not support the requested target
(powerpc-unknown-linux-gnuspe), yet the Make logic still attempted to add it.

The logic has been corrected by performing rustup toolchain and target
availability checks at runtime in the shell, ensuring that targets are only
added when supported by the selected Rust toolchain and avoiding invalid
rustup invocations.

Author: th0ma7

mk/spksrc.tc-rust.mk

@@ -103,13 +103,20 @@ rustc_target: $(PRE_RUSTC_TARGET) $(TC_LOCAL_VARS_RUST)
 	$(MSG) "rustup default $(RUSTUP_DEFAULT_TOOLCHAIN)" ; \
 	rustup default $(RUSTUP_DEFAULT_TOOLCHAIN) ; \
 	flock -u 5
-ifeq ($(TC_RUSTUP_TOOLCHAIN),$(RUSTUP_DEFAULT_TOOLCHAIN))
-	@$(MSG) "rustup target add $(RUST_TARGET)"
-	rustup override set $(RUSTUP_DEFAULT_TOOLCHAIN)
-	rustup target add $(RUST_TARGET)
-	rustup show
-else
-	@$(MSG) "Target $(RUST_TARGET) unavailable..."
+	@$(MSG) "Checking Rust target $(RUST_TARGET) availability"
+	@if rustup target list --toolchain $(TC_RUSTUP_TOOLCHAIN) 2>/dev/null | grep -q "^$(RUST_TARGET) (installed)" ; then \
+	   $(MSG) "Rust target already installed for $(TC_RUSTUP_TOOLCHAIN)" ; \
+	   rustup show ; \
+	elif rustup target list --toolchain $(TC_RUSTUP_TOOLCHAIN) 2>/dev/null | grep -q "^$(RUST_TARGET)" ; then \
+	   $(MSG) "Installing Rust target $(RUST_TARGET) for $(TC_RUSTUP_TOOLCHAIN)" ; \
+	   $(MSG) "rustup override set $(RUSTUP_DEFAULT_TOOLCHAIN)" ; \
+	   rustup override set $(RUSTUP_DEFAULT_TOOLCHAIN) ; \
+	   $(MSG) "rustup target add $(RUST_TARGET)" ; \
+	   rustup target add $(RUST_TARGET) ; \
+	   rustup show ; \
+	else \
+	   $(MSG) "Target $(RUST_TARGET) unavailable for $(TC_RUSTUP_TOOLCHAIN)" ; \
+	fi
 ifeq ($(RUST_BUILD_TOOLCHAIN),1)
 	@$(MSG) "Build rust target $(RUST_TARGET) from sources"
 	@$(MSG) "Building Tier-3 rust target: $(RUST_TARGET)"
@@ -133,7 +140,6 @@ endif
 	@$(MSG) "Building Tier 3 rust target: $(RUST_TARGET) - stage$(RUSTUP_DEFAULT_TOOLCHAIN_STAGE) complete"
 	rustup show
 endif
-endif
 
 post_rustc_target: $(RUSTC_TARGET)