LibC: Implement various things to get GNU bc building and running.

Looks like that's all we needed, and bc now runs. :^)

LibC/stdlib.h

@@ -21,6 +21,7 @@ void abort() __NORETURN;
 char* ptsname(int fd);
 int ptsname_r(int fd, char* buffer, size_t);
 int abs(int);
+int system(const char* command);
 
 #define RAND_MAX 32767
 int rand();