riscv: Fix feenvupdate with FE_DFL_ENV (BZ 31022)

libc_feupdateenv_riscv should check for FE_DFL_ENV, similar to
libc_fesetenv_riscv.

Also extend the test-fenv.c to test fenvupdate.

Checked on riscv under qemu-system.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>