Fix target_abi in sparc-unknown-none-elf

This was previously set to `target_abi = "elf"`, but `elf` is not used
elsewhere as a target ABI (even though there's many targets that have it
in their name).

Author: madsmtm

compiler/rustc_target/src/spec/targets/sparc_unknown_none_elf.rs

@@ -7,7 +7,7 @@ pub(crate) fn target() -> Target {
         linker: Some("sparc-elf-gcc".into()),
         endian: Endian::Big,
         cpu: "v7".into(),
-        abi: "elf".into(),
+        abi: "".into(),
         max_atomic_width: Some(32),
         atomic_cas: true,
         panic_strategy: PanicStrategy::Abort,