Rollup merge of #130334 - madsmtm:macos-sdkroot-ignore, r=jieyouxu

Fix `SDKROOT` ignore on macOS

`rustc` has code to detect when `SDKROOT` is obviously set for the wrong platform, so that it can choose to ignore it. This is a pretty important feature for Cargo build scripts and proc macros, since you will often have `SDKROOT` set to an iOS platform there.

However, the code was checking for an old SDK version name `"macosx10.15"` that was previously configured by `add_apple_sdk`, but nowadays configured to the correct `"macosx"`. I think this error was introduced in part #77202 and in #100286.

Fixes part of #80817 (linking with `-Clinker=ld` now works), though more work is still needed in this area, see also #129432.

``@rustbot`` label O-macos A-cross

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -3044,7 +3044,7 @@ fn get_apple_sdk_root(sdk_name: &str) -> Result<String, errors::AppleSdkRootErro
             "iphonesimulator"
                 if sdkroot.contains("iPhoneOS.platform") || sdkroot.contains("MacOSX.platform") => {
             }
-            "macosx10.15"
+            "macosx"
                 if sdkroot.contains("iPhoneOS.platform")
                     || sdkroot.contains("iPhoneSimulator.platform") => {}
             "watchos"