vendor/libc: a bunch of additions

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

vendor/libc/include/stdlib.h

@@ -1,3 +1,9 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#pragma once
+
 #include <stddef.h>
 
 void *malloc(size_t size);
@@ -17,3 +23,22 @@ long long atoll(const char *);
 double atof(const char *);
 
 long strtol(const char *, char **, int);
+double strtod(const char *, char **);
+
+void abort();
+void exit(int exit_code);
+
+#define ATEXIT_MAX 32
+
+int atexit(typeof(void (void)) *);
+
+typedef struct {
+	long int quot;
+	long int rem;
+} ldiv_t;
+
+ldiv_t ldiv(long int number, long int denom);
+
+#ifdef __cplusplus
+}
+#endif