Move _float80ToStringImpl to the x86_64-specific ABI checker file. (#85002)

Author: stephentyrone

test/api-digester/Inputs/stability-stdlib-abi-without-asserts-x86_64.txt

@@ -1 +1,5 @@
 // ABI additions for x86_64 only.
+
+// Float-to-String conversions were reimplemented in Swift
+// The legacy C entry points now look like newly introduced Swift functions
+Func _float80ToStringImpl(_:_:_:_:) is a new API without '@available'

test/api-digester/stability-stdlib-abi-without-asserts.test

@@ -872,6 +872,5 @@ Func type(of:) has been removed
 // The legacy C entry points now look like newly introduced Swift functions
 Func _float32ToStringImpl(_:_:_:_:) is a new API without '@available'
 Func _float64ToStringImpl(_:_:_:_:) is a new API without '@available'
-Func _float80ToStringImpl(_:_:_:_:) is a new API without '@available'
 
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)