Fold aarch64 feature +fp into +neon

Arm's FEAT_FP and Feat_AdvSIMD describe the same thing on AArch64:
The Neon unit, which handles both floating point and SIMD instructions.
Moreover, a configuration for AArch64 must include both or neither.
Arm says "entirely proprietary" toolchains may omit floating point:
https://developer.arm.com/documentation/102374/0101/Data-processing---floating-point
In the Programmer's Guide for Armv8-A, Arm says AArch64 can have
both FP and Neon or neither in custom implementations:
https://developer.arm.com/documentation/den0024/a/AArch64-Floating-point-and-NEON

In "Bare metal boot code for Armv8-A", enabling Neon and FP
is just disabling the same trap flag:
https://developer.arm.com/documentation/dai0527/a

In an unlikely future where "Neon and FP" become unrelated,
we can add "[+-]fp" as its own feature flag.
Until then, we can simplify programming with Rust on AArch64 by
folding both into "[+-]neon", which is valid as it supersets both.

"[+-]neon" is retained for niche uses such as firmware, kernels,
"I just hate floats", and so on.

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -43,10 +43,8 @@ const ARM_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
 ];
 
 const AARCH64_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
-    // FEAT_AdvSimd
+    // FEAT_AdvSimd & FEAT_FP
     ("neon", None),
-    // FEAT_FP
-    ("fp", None),
     // FEAT_FP16
     ("fp16", None),
     // FEAT_SVE
@@ -143,7 +141,6 @@ const AARCH64_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
 ];
 
 const AARCH64_TIED_FEATURES: &[&[&str]] = &[
-    &["fp", "neon"],   // Silicon always has both, so avoid needless complications
     &["paca", "pacg"], // Together these represent `pauth` in LLVM
 ];