test unstable values of -C linker-flavor=gcc

Author: lqd

src/test/ui/linkers/unstable_linker_flavor.lld.stderr

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+error: the `gcc:*` linker flavor is unstable, the `-Z unstable-options` flag must also be passed to use it
+

src/test/ui/linkers/unstable_linker_flavor.other.stderr

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+error: the `gcc:*` linker flavor is unstable, the `-Z unstable-options` flag must also be passed to use it
+

src/test/ui/linkers/unstable_linker_flavor.rs

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+// check-fail
+// revisions: lld other
+// [lld] compile-flags: -C linker-flavor=gcc:lld
+// [other] compile-flags: -C linker-flavor=gcc:other
+
+// Test ensuring that the unstable `gcc:*` values of the stable `-C linker-flavor` flag require
+// using `-Z unstable options`
+
+fn main() {}