ArmPkg: Fix Ecc error 5007 in CompilerIntrinsicsLib

This patch fixes the following Ecc reported error:
There should be no initialization of a variable as
part of its declaration

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

ArmPkg/Library/CompilerIntrinsicsLib/memset.c

@@ -12,7 +12,9 @@ typedef __SIZE_TYPE__ size_t;
 static __attribute__((__used__))
 void *__memset(void *s, int c, size_t n)
 {
-  unsigned char *d = s;
+  unsigned char *d;
+
+  d = s;
 
   while (n-- != 0) {
     *d++ = c;