For a simple random number generator running on an embedded CPU we're unlikely to be very concerned with cycle lengths -- a cycle of 2^32-1 is perfectly acceptable. Marsaglia suggests coefficients (13, 17, 5) for a 32-bit generator.

random.fs

variable seed
7 seed !

: random    ( -- x )  \ return a 32-bit random number x
    seed @
    dup 13 lshift xor
    dup 17 rshift xor
    dup 5  lshift xor
    dup seed !
;

: setseed   ( x -- )  \ seed the RNG with x
    dup 0= or         \ map 0 to -1
    seed !
;