Hi all,

From the `force_reg` description comment I see the returned register
should not be modified, thus IIUC should not be used as a GEN_FCN
target.

Assuming my interpretation is correct this fix this case inside
`aarch64_general_expand_builtin` while expanding expanding the
`__jcvt` intrinsic.  If is not the case please discard.

Regtested and bootsraped on aarch64-linux-gnu.

  Andrea