Implement rand/srand as 16-bit xorshift

This is a fairly high quality rand; it has a low period (lower than that
required by POSIX), but few other defects given its performance.

mos-platform/common/c/stdlib.c

@@ -479,3 +479,19 @@ strtoui(const char *__restrict__ nptr, char **__restrict__ endptr, int base) {
 
   return (sign == '+' || is_error) ? rc : -rc;
 }
+
+// A 16-bit xorshift generator with constants well suited to 8-bit systems.
+// Values found by George Marsaglia for the Z80:
+//   http://www.retroprogramming.com/2017/07/xorshift-pseudorandom-numbers-in-z80.html
+
+static unsigned seed = 1;
+
+__attribute__((weak)) int rand(void) {
+  unsigned x = seed;
+  x ^= x << 7;
+  x ^= x >> 9;
+  x ^= x << 8;
+  return seed = x;
+}
+
+__attribute__((weak)) void srand(unsigned s) { seed = s; }

mos-platform/common/include/stdlib.h

@@ -71,6 +71,9 @@ int strtoi(const char *__restrict__ nptr, char **__restrict endptr, int base);
 unsigned int strtoui(const char *__restrict__ nptr, char **__restrict endptr,
                      int base);
 
+int rand(void);
+void srand(unsigned seed);
+
 // clang-format off
 /**
 Simple malloc/free implementation.