update JSON target spec to declare softfloat ABI

Author: RalfJung

src/tools/miri/tests/x86_64-unknown-kernel.json

@@ -10,6 +10,7 @@
   "vendor": "unknown",
   "linker": "rust-lld",
   "linker-flavor": "gnu-lld",
+  "rustc-abi": "x86-softfloat",
   "features": "-mmx,-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2,+soft-float",
   "dynamic-linking": false,
   "executables": true,