[newlib-cygwin] Fix memccpy to handle end char >= x80

[Jeff Johnston]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=ad3f9820b16a3dc5ea6237106436f565fcb2ed3e

commit ad3f9820b16a3dc5ea6237106436f565fcb2ed3e
Author: CompilerAI Research Group <bugs@compiler.ai>
Date:   Tue Jan 3 14:45:37 2023 -0500

    Fix memccpy to handle end char >= x80
    
    - use unsigned char variables for optimized version of memccpy

Diff:
---
 newlib/libc/string/memccpy.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/newlib/libc/string/memccpy.c b/newlib/libc/string/memccpy.c
index 1f5f55c50..6757cb34c 100644
--- a/newlib/libc/string/memccpy.c
+++ b/newlib/libc/string/memccpy.c
@@ -80,11 +80,11 @@ memccpy (void *__restrict dst0,
   return ptr;
 #else
   void *ptr = NULL;
-  char *dst = dst0;
-  const char *src = src0;
+  unsigned char *dst = dst0;
+  const unsigned char *src = src0;
   long *aligned_dst;
   const long *aligned_src;
-  char endchar = endchar0 & 0xff;
+  unsigned char endchar = endchar0 & 0xff;
 
   /* If the size is small, or either SRC or DST is unaligned,
      then punt into the byte copy loop.  This should be rare.  */