test/linkage-visibility: Ignore on musl targets

DynamicLibrary uses libc's dlsym() function internally to find symbols.
Some implementations of dlsym(), like musl's, only look at dynamically-
exported symbols, as found in shared libraries. To also export symbols
from the main executable, we would need to pass --export-dynamic to the
linker. Since this flag isn't available everywhere, ignore the test for
now.

src/test/run-pass-fulldeps/auxiliary/linkage-visibility.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-musl - dlsym doesn't see symbols without "-C link-arg=-Wl,--export-dynamic"
+
 #![feature(rustc_private)]
 
 // We're testing linkage visibility; the compiler warns us, but we want to