clean up internal comments about float semantics

- remove an outdated FIXME
- add reference to floating-point semantics issue

Co-authored-by: Jubilee <workingjubilee@gmail.com>

library/core/src/num/f16.rs

@@ -435,6 +435,7 @@ impl f16 {
         // WASM, see llvm/llvm-project#96437). These are platforms bugs, and Rust will misbehave on
         // such platforms, but we can at least try to make things seem as sane as possible by being
         // careful here.
+        // see also https://github.com/rust-lang/rust/issues/114479
         if self.is_infinite() {
             // Thus, a value may compare unequal to infinity, despite having a "full" exponent mask.
             FpCategory::Infinite

library/core/src/num/f32.rs

@@ -662,10 +662,7 @@ impl f32 {
         // hardware flushes subnormals to zero. These are platforms bugs, and Rust will misbehave on
         // such hardware, but we can at least try to make things seem as sane as possible by being
         // careful here.
-        //
-        // FIXME(jubilee): Using x87 operations is never necessary in order to function
-        // on x86 processors for Rust-to-Rust calls, so this issue should not happen.
-        // Code generation should be adjusted to use non-C calling conventions, avoiding this.
+        // see also https://github.com/rust-lang/rust/issues/114479
         if self.is_infinite() {
             // A value may compare unequal to infinity, despite having a "full" exponent mask.
             FpCategory::Infinite

library/core/src/num/f64.rs

@@ -660,10 +660,7 @@ impl f64 {
         // float semantics Rust relies on: x87 uses a too-large exponent, and some hardware flushes
         // subnormals to zero. These are platforms bugs, and Rust will misbehave on such hardware,
         // but we can at least try to make things seem as sane as possible by being careful here.
-        //
-        // FIXME(jubilee): Using x87 operations is never necessary in order to function
-        // on x86 processors for Rust-to-Rust calls, so this issue should not happen.
-        // Code generation should be adjusted to use non-C calling conventions, avoiding this.
+        // see also https://github.com/rust-lang/rust/issues/114479
         //
         // Thus, a value may compare unequal to infinity, despite having a "full" exponent mask.
         // And it may not be NaN, as it can simply be an "overextended" finite value.

tests/ui/numbers-arithmetic/issue-105626.rs

@@ -11,6 +11,7 @@ fn main() {
     assert_ne!((n as f64) as f32, n as f32);
 
     // FIXME: these assertions fail if only x87 is enabled
+    // see also https://github.com/rust-lang/rust/issues/114479
     assert_eq!(n as i64 as f32, r);
     assert_eq!(n as u64 as f32, r);
 }