fix `+soft-float` in x86_64-xen-pv target

[n0toose]

See: rust-osdev/bootloader#492

This is the resulting output when attempting to build using x86_64-xen-pv.json:

$ cargo build --target x86_64-xen-pv.json -Zbuild-std=core -Zbuild-std-features=compiler-builtins-mem
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/home/myuseraccount/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc - --crate-name ___ --print=file-names --target /tmp/xen/oxerun/x86_64-xen-pv.json --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit status: 1)
  --- stderr
  error: error loading target specification: target feature `soft-float` is incompatible with the ABI but gets enabled in target spec
    |
    = help: run `rustc --print target-list` for a list of built-in targets

[n0toose]

Just for the record, I tried to apply the fix from the PR mentioned above "bluntly", but I didn't come far with that:

> cargo build --target x86_64-xen-pv.json -Zbuild-std=core
   Compiling core v0.0.0 (/var/home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core)
   Compiling compiler_builtins v0.1.160
warning: target json file contains unused fields: is-builtin

   Compiling rustc-std-workspace-core v1.99.0 (/var/home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-core)
error: data-layout for target `x86_64-xen-pv-6424415598074553043`, `e-m:e-i64:64-f80:128-n8:16:32:64-S128`, differs from LLVM target's `x86_64-unknown-none` default layout, `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128`

warning: `rustc-std-workspace-core` (lib) generated 1 warning (1 duplicate)
error: could not compile `rustc-std-workspace-core` (lib) due to 1 previous error; 1 warning emitted
warning: build failed, waiting for other jobs to finish...
warning: `core` (lib) generated 1 warning
error: could not compile `core` (lib) due to 1 previous error; 1 warning emitted
warning: `compiler_builtins` (lib) generated 1 warning (1 duplicate)
error: could not compile `compiler_builtins` (lib) due to 1 previous error; 1 warning emitted

[n0toose]

Potentially indirectly related: rust-osdev/bootloader@2fe7c2e