[libc][RISC-V] Temporarily disable dirent (#207867)

The RISC-V 32 QEMU buildbot is failing due to missing SYS_fcntl, which
is needed for fdopendir. Since that syscall is referenced in the shared
dir code it's easiest to just disable all the dirent functions for now.

GitOrigin-RevId: 8f3e6197f676831794bfa7a7b4619c59c7915c42