gc-random: Replace implementation with call to getrandom.
* lib/gc-gnulib.c [GNULIB_GC_RANDOM]: Replace #include's with
those needed for getrandom.
(gc_init): Remove old randomness code.
(gc_done): Likewise.
(randomize): Rewrite using getrandom, inspired by getentropy.
* m4/gc-random.m4: Remove file.
* modules/crypto/gc-random: Drop gc-random.m4, gl_GC_RANDOM, and
LIB_GC_RANDOM. Add conditional dependency on getrandom.
* modules/crypto/gc-tests (test_gc_LDADD): Drop LIB_GC_RANDOM.