fixes remaining ptr2cstring warnings on version-1-6 (#20861)

lib/pure/os.nim

@@ -1146,7 +1146,7 @@ when defined(windows) and not weirdTarget:
     template findNextFile(a, b: untyped): untyped = findNextFileA(a, b)
     template getCommandLine(): untyped = getCommandLineA()
 
-    template getFilename(f: untyped): untyped = $cstring(addr f.cFileName)
+    template getFilename(f: untyped): untyped = $cast[cstring](addr f.cFileName)
 
   proc skipFindData(f: WIN32_FIND_DATA): bool {.inline.} =
     # Note - takes advantage of null delimiter in the cstring
@@ -2337,7 +2337,7 @@ iterator walkDir*(dir: string; relative = false, checkDir = false):
         while true:
           var x = readdir(d)
           if x == nil: break
-          var y = $cstring(addr x.d_name)
+          var y = $cast[cstring](addr x.d_name)
           if y != "." and y != "..":
             var s: Stat
             let path = dir / y

lib/system/formatfloat.nim

@@ -44,7 +44,7 @@ proc writeFloatToBufferSprintf*(buf: var array[65, char]; value: BiggestFloat):
   ##
   ## returns the amount of bytes written to `buf` not counting the
   ## terminating '\0' character.
-  var n: int = c_sprintf(addr buf, "%.16g", value)
+  var n: int = c_sprintf(cast[cstring](addr buf), "%.16g", value)
   var hasDot = false
   for i in 0..n-1:
     if buf[i] == ',':
@@ -85,7 +85,7 @@ proc addFloatRoundtrip*(result: var string; x: float | float32) =
   else:
     var buffer {.noinit.}: array[65, char]
     let n = writeFloatToBufferRoundtrip(buffer, x)
-    result.addCstringN(cstring(buffer[0].addr), n)
+    result.addCstringN(cast[cstring](buffer[0].addr), n)
 
 proc addFloatSprintf*(result: var string; x: float) =
   when nimvm:
@@ -93,7 +93,7 @@ proc addFloatSprintf*(result: var string; x: float) =
   else:
     var buffer {.noinit.}: array[65, char]
     let n = writeFloatToBufferSprintf(buffer, x)
-    result.addCstringN(cstring(buffer[0].addr), n)
+    result.addCstringN(cast[cstring](buffer[0].addr), n)
 
 proc nimFloatToString(a: float): cstring =
   ## ensures the result doesn't print like an integer, i.e. return 2.0, not 2

tests/system/tostring.nim

@@ -47,7 +47,7 @@ import strutils
 
 let arr = ['H','e','l','l','o',' ','W','o','r','l','d','!','\0']
 doAssert $arr == "['H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!', '\\x00']"
-doAssert $cast[cstring](addr arr) == "Hello World!"
+doAssert $cast[cstring](unsafeAddr arr) == "Hello World!"
 
 proc takes(c: cstring) =
   doAssert c == cstring""