Try to fix up the availability of the Swift and C projections of the legacy ABI

Author: tbkka

stdlib/public/core/FloatingPointToString.swift

@@ -130,7 +130,7 @@
 #if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
 
 // Support Legacy ABI on top of new implementation
-@available(SwiftStdlib 6.2, *)
+@available(SwiftStdlib 5.3, *)
 @_silgen_name("swift_float16ToString")
 public func _float16ToStringImpl(
   _ textBuffer: UnsafeMutablePointer<UTF8.CodeUnit>,
@@ -420,7 +420,8 @@ internal func _Float16ToASCII(
 
 // Support Legacy ABI on top of new implementation
 @_silgen_name("swift_float32ToString")
-public func _float32ToStringImpl(
+@usableFromInline
+func _float32ToStringImpl(
   _ textBuffer: UnsafeMutablePointer<UTF8.CodeUnit>,
   _ bufferLength: UInt,
   _ value: Float32,
@@ -672,7 +673,8 @@ internal func _Float32ToASCII(
 
 // Support Legacy ABI on top of new implementation
 @_silgen_name("swift_float64ToString")
-public func _float64ToStringImpl(
+@usableFromInline
+func _float64ToStringImpl(
   _ textBuffer: UnsafeMutablePointer<UTF8.CodeUnit>,
   _ bufferLength: UInt,
   _ value: Float64,
@@ -1179,7 +1181,8 @@ internal func _Float64ToASCII(
 
 // Support Legacy ABI on top of new implementation
 @_silgen_name("swift_float80ToString")
-internal func _float80ToStringImpl(
+@usableFromInline
+func _float80ToStringImpl(
   _ textBuffer: UnsafeMutablePointer<UTF8.CodeUnit>,
   _ bufferLength: UInt,
   _ value: Float80,