add kernel target for RustyHermit

Currently, we are thinking to use *-unknown-none targets instead
to define for every platform our own one (see hermit-os/hermit-rs#197).
However, the current target aarch64-unknown-none-softfloat doesn't support
dynamic relocation. Our kernel uses this feature and consequently
we define a new target aarch64-unknown-hermitkernel to support it.

compiler/rustc_target/src/spec/aarch64_unknown_hermit.rs

@@ -3,6 +3,7 @@ use crate::spec::Target;
 pub fn target() -> Target {
     let mut base = super::hermit_base::opts();
     base.max_atomic_width = Some(128);
+    base.features = "+strict-align,+neon,+fp-armv8".to_string();
 
     Target {
         llvm_target: "aarch64-unknown-hermit".to_string(),

compiler/rustc_target/src/spec/aarch64_unknown_hermitkernel.rs

@@ -0,0 +1,16 @@
+use crate::spec::Target;
+
+pub fn target() -> Target {
+    let mut base = super::hermit_kernel_base::opts();
+    base.max_atomic_width = Some(128);
+    base.abi = "softfloat".to_string();
+    base.features = "+strict-align,-neon,-fp-armv8".to_string();
+
+    Target {
+        llvm_target: "aarch64-unknown-hermit".to_string(),
+        pointer_width: 64,
+        data_layout: "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128".to_string(),
+        arch: "aarch64".to_string(),
+        options: base,
+    }
+}