random-util: allow RDRAND to be used in 32-bit x86 binaries

Rename rdrand64 to rdrand, and switch from uint64_t to unsigned long.
This produces code that will compile/assemble on both x86-64 and x86-32.

This could be useful when running a 32-bit copy of systemd on a modern
Intel processor.

RDRAND is inherently arch-specific, so relying on the compiler-defined
'long' type seems reasonable.

src/basic/random-util.h

@@ -30,4 +30,4 @@ static inline uint32_t random_u32(void) {
         return u;
 }
 
-int rdrand64(uint64_t *ret);
+int rdrand(unsigned long *ret);