WASI: Fix fd_fdstat_set_rights to set the rights.
WASI: Fix fd_fdstat_set_rights to set the rights.

After checking that no new rights are being added, actually set the
rights, which may be a subset of the original rights.

via @sunfishcode

fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet,fastly/lucet