change rustc-abi in custom targets to x86-softfloat

This is a direct port of the Tom Dohrmann's PR:
rust-osdev/bootloader#492

Which, in turn, "fixes a nightly breakage introduced by":
rust-lang/rust#136146

Author: n0toose

oxerun/x86_64-xen-pv.json

@@ -1,12 +1,13 @@
 {
   "arch": "x86_64",
   "cpu": "x86-64",
-  "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
+  "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
   "disable-redzone": true,
   "dynamic-linking": false,
   "env": "",
   "executables": true,
   "features": "-mmx,-sse,+soft-float",
+  "rustc-abi": "x86-softfloat",
   "has-rpath": false,
   "is-builtin": false,
   "linker": "rust-lld",