Prioritize RUSTC_LINKER over target->cross-compile-prefix table.

Rationale is that if rustc is explicitly instructed to use a specific
linker, using it as base for cross-compile prefix would be better
aligned with user expectations.

Author: dot-asm

src/lib.rs

@@ -2721,13 +2721,13 @@ impl Build {
     }
 
     fn prefix_for_target(&self, target: &str) -> Option<String> {
-        // Put aside RUSTC_LINKER's prefix to be used as last resort
+        // Put aside RUSTC_LINKER's prefix to be used as second choice, after CROSS_COMPILE
         let rustc_linker = self.getenv("RUSTC_LINKER").unwrap_or("".to_string());
         // let linker_prefix = rustc_linker.strip_suffix("-gcc"); // >=1.45.0
         let linker_prefix = if rustc_linker.len() > 4 {
             let (prefix, suffix) = rustc_linker.split_at(rustc_linker.len() - 4);
             if suffix == "-gcc" {
-                Some(prefix)
+                Some(prefix.to_string())
             } else {
                 None
             }
@@ -2737,7 +2737,7 @@ impl Build {
         // CROSS_COMPILE is of the form: "arm-linux-gnueabi-"
         let cc_env = self.getenv("CROSS_COMPILE");
         let cross_compile = cc_env.as_ref().map(|s| s.trim_end_matches('-').to_owned());
-        cross_compile.or(match &target[..] {
+        cross_compile.or(linker_prefix).or(match &target[..] {
             // Note: there is no `aarch64-pc-windows-gnu` target, only `-gnullvm`
             "aarch64-pc-windows-gnullvm" => Some("aarch64-w64-mingw32"),
             "aarch64-uwp-windows-gnu" => Some("aarch64-w64-mingw32"),
@@ -2850,7 +2850,7 @@ impl Build {
             ]), // explicit None if not found, so caller knows to fall back
             "x86_64-unknown-linux-musl" => Some("musl"),
             "x86_64-unknown-netbsd" => Some("x86_64--netbsd"),
-            _ => linker_prefix,
+            _ => None,
         }
         .map(|x| x.to_owned()))
     }