Add more const-correct C++ overloads.

libc++ actually provides overloads for all the standard C library stuff,
so we just need to handle the POSIX and GNU extensions, of which there
are just two more: memrchr and strcasestr.

Bug: http://b/22768375
Test: builds
Change-Id: Ie9ed1fbcc794e14a0c9bba13b5307ad677949613
diff --git a/libc/include/string.h b/libc/include/string.h
index 33ef468..d409ba8 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -43,7 +43,12 @@
 
 void* memccpy(void* __dst, const void* __src, int __stop_char, size_t __n);
 void* memchr(const void* __s, int __ch, size_t __n) __attribute_pure__ __overloadable __RENAME_CLANG(memchr);
+#if defined(__cplusplus)
+extern "C++" void* memrchr(void* __s, int __ch, size_t __n) __RENAME(memrchr) __attribute_pure__;
+extern "C++" const void* memrchr(const void* __s, int __ch, size_t __n) __RENAME(memrchr) __attribute_pure__;
+#else
 void* memrchr(const void* __s, int __ch, size_t __n) __attribute_pure__ __overloadable __RENAME_CLANG(memrchr);
+#endif
 int memcmp(const void* __lhs, const void* __rhs, size_t __n) __attribute_pure__;
 void* memcpy(void*, const void*, size_t)
         __overloadable __RENAME_CLANG(memcpy);
@@ -81,7 +86,12 @@
 char* strdup(const char* __s);
 
 char* strstr(const char* __haystack, const char* __needle) __attribute_pure__;
+#if defined(__cplusplus)
+extern "C++" char* strcasestr(char*, const char*) __RENAME(strcasestr) __attribute_pure__;
+extern "C++" const char* strcasestr(const char*, const char*) __RENAME(strcasestr) __attribute_pure__;
+#else
 char* strcasestr(const char* __haystack, const char* __needle) __attribute_pure__;
+#endif
 char* strtok(char* __s, const char* __delimiter);
 char* strtok_r(char* __s, const char* __delimiter, char** __pos_ptr);