Avoid shift wider than unsigned HOST_WIDE_INT on unsigned integer exponentiation.

Avoid shift wider than unsigned HOST_WIDE_INT on unsigned integer exponentiation.

this patch is a variation of Jakub's patch in the PR, which
avoids overflow on the mask used for exponentiation and
fixes unsigned HOST_WIDE_INT.  I tried testing this on
a POWER machine, but --with-build-config=bootstrap-ubsan
fails bootstrap there.

gcc/fortran/ChangeLog:

	PR fortran/118862
	* trans-expr.cc (gfc_conv_cst_int_power): Use functions for
	unsigned wide integer.
	(gfc_conv_cst_uint_power): Avoid generating the mask if it would
	overflow an unsigned HOST_WIDE_INT.  Format fixes.