string: Fix wrong comparison semantics

Chars are signed for x86 -- correct the comparison semantics.

Change-Id: I2049e98eb063c0b4e83ea973d3fcae49c6817dde
Author: Liubov Dmitrieva <liubov.dmitrieva@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
diff --git a/libc/string/memrchr.c b/libc/string/memrchr.c
index a533fef..aeb5643 100644
--- a/libc/string/memrchr.c
+++ b/libc/string/memrchr.c
@@ -35,10 +35,10 @@
         const char*  q = p + n;
 
         while (1) {
-            q--; if (q < p || q[0] == c) break;
-            q--; if (q < p || q[0] == c) break;
-            q--; if (q < p || q[0] == c) break;
-            q--; if (q < p || q[0] == c) break;
+            q--; if (q < p || q[0] == (char) c) break;
+            q--; if (q < p || q[0] == (char) c) break;
+            q--; if (q < p || q[0] == (char) c) break;
+            q--; if (q < p || q[0] == (char) c) break;
         }
         if (q >= p)
             return (void*)q;