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).

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,

tests/ui/check-cfg/well-known-values.stderr

@@ -129,7 +129,7 @@ warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
 LL |     target_abi = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_abi` are: ``, `abi64`, `abiv2`, `abiv2hf`, `eabi`, `eabihf`, `elf`, `fortanix`, `ilp32`, `llvm`, `macabi`, `sim`, `softfloat`, `spe`, `uwp`, `vec-extabi`, and `x32`
+   = note: expected values for `target_abi` are: ``, `abi64`, `abiv2`, `abiv2hf`, `eabi`, `eabihf`, `fortanix`, `ilp32`, `llvm`, `macabi`, `sim`, `softfloat`, `spe`, `uwp`, `vec-extabi`, and `x32`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`