Use the right TLS model for CloudABI.

CloudABI doesn't do dynamic linking. For this reason, there is no need
to handle any other TLS model than local-exec. CloudABI's C library
doesn't provide a __tls_get_addr() function to do Dynamic TLS.

By forcing local-exec to be used here, we ensure that we don't generate
function calls to __tls_get_addr().

src/librustc_back/target/cloudabi_base.rs

@@ -27,6 +27,7 @@ pub fn opts() -> TargetOptions {
         linker_is_gnu: true,
         pre_link_args: args,
         position_independent_executables: true,
+        tls_model: "local-exec".to_string(),
         relro_level: RelroLevel::Full,
         exe_allocation_crate: super::maybe_jemalloc(),
         .. Default::default()