add test ensuring -Zgcc-ld=$linker and -Clinker-flavor=gcc:$linker match

currently only "lld" is usable in this context

Author: lqd

src/test/ui/linkers/gcc_ld_mismatch.rs

@@ -0,0 +1,7 @@
+// check-fail
+// compile-flags: -Zgcc-ld=lld -Clinker-flavor=gcc:not_lld -Zunstable-options
+
+// Test ensuring that until the unstable flag is removed (if ever), if both the linker-flavor and
+// `gcc-ld` flags are used, they ask for the same linker.
+
+fn main() {}

src/test/ui/linkers/gcc_ld_mismatch.stderr

@@ -0,0 +1,2 @@
+error: `-Zgcc-ld=lld` and `-Clinker-flavor` differ in their linker choice. The latter should be `-Clinker-flavor=gcc:lld`
+