vendor/libc: a bunch of additions

All these additions are to allow Dear ImGui to be compiled natively.

vendor/libc/include/string.h

@@ -1,9 +1,16 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#pragma once
+
 #include <stddef.h>
 
 void *memcpy(void *, const void *, size_t);
 void *memset(void *, int, size_t);
 void *memmove(void *, void *, size_t);
 int memcmp(const void *, const void *, size_t);
+void *memchr(const void *, int, size_t);
 
 unsigned long strlen(const char *str);
 
@@ -19,3 +26,7 @@ int strcmp(const char *, const char *);
 int strncmp(const char *, const char *, size_t);
 
 char *strstr(const char *, const char *);
+
+#ifdef __cplusplus
+}
+#endif