fix c_memchr, c_strstr definitions (#24587)

One correct definition is enough

lib/std/formatfloat.nim

@@ -12,7 +12,7 @@
 when defined(nimPreviewSlimSystem):
   import std/assertions
 
-proc c_memcpy(a, b: pointer, size: csize_t): pointer {.importc: "memcpy", header: "<string.h>", discardable.}
+from system/ansi_c import c_memcpy
 
 proc addCstringN(result: var string, buf: cstring; buflen: int) =
   # no nimvm support needed, so it doesn't need to be fast here either

lib/std/private/strimpl.nim

@@ -75,11 +75,7 @@ template endsWithImpl*[T: string | cstring](s, suffix: T) =
 func cmpNimIdentifier*[T: string | cstring](a, b: T): int =
   cmpIgnoreStyleImpl(a, b, true)
 
-func c_memchr(cstr: pointer, c: char, n: csize_t): pointer {.
-              importc: "memchr", header: "<string.h>".}
-func c_strstr(haystack, needle: cstring): cstring {.
-  importc: "strstr", header: "<string.h>".}
-
+from system/ansi_c import c_memchr, c_strstr
 
 func find*(s: cstring, sub: char, start: Natural = 0, last = 0): int =
   ## Searches for `sub` in `s` inside the range `start..last` (both ends included).
@@ -91,7 +87,7 @@ func find*(s: cstring, sub: char, start: Natural = 0, last = 0): int =
   let last = if last == 0: s.high else: last
   let L = last-start+1
   if L > 0:
-    let found = c_memchr(s[start].unsafeAddr, sub, cast[csize_t](L))
+    let found = c_memchr(s[start].unsafeAddr, cint(sub), cast[csize_t](L))
     if not found.isNil:
       return cast[int](found) -% cast[int](s)
   return -1

lib/std/syncio.nim

@@ -15,6 +15,8 @@ import std/formatfloat
 when defined(windows):
   import std/widestrs
 
+from system/ansi_c import c_memchr
+
 # ----------------- IO Part ------------------------------------------------
 type
   CFile {.importc: "FILE", header: "<stdio.h>",
@@ -401,9 +403,6 @@ proc readLine*(f: File, line: var string): bool {.tags: [ReadIOEffect],
   ## `false` is returned `line` contains no new data.
   result = false
 
-  proc c_memchr(s: pointer, c: cint, n: csize_t): pointer {.
-    importc: "memchr", header: "<string.h>".}
-
   when defined(windows):
     proc readConsole(hConsoleInput: FileHandle, lpBuffer: pointer,
                      nNumberOfCharsToRead: int32,
@@ -491,7 +490,7 @@ proc readLine*(f: File, line: var string): bool {.tags: [ReadIOEffect],
       checkErr(f)
       break
 
-    let m = c_memchr(addr line[pos], '\L'.ord, cast[csize_t](sp))
+    let m = c_memchr(addr line[pos], cint('\L'), cast[csize_t](sp))
     if m != nil:
       # \l found: Could be our own or the one by fgets, in any case, we're done
       var last = cast[int](m) - cast[int](addr line[0])