arm: big-endian issue in gen_cpymem_ldrd_strd [PR105981]

arm: big-endian issue in gen_cpymem_ldrd_strd [PR105981]

The code in gen_cpymem_ldrd_strd has been incorrect for big-endian
since r230663.  The problem is that we use gen_lowpart, etc. to split
the 64-bit quantity, but fail to account for the fact that these
routines are really dealing with 64-bit /values/ and in big-endian the
ordering of the sub-registers changes.

To fix this, I've renamed the conceptually misnamed low_reg and hi_reg
as first_reg and second_reg, and then used different logic for
big-endian targets to initialize these values.  This makes the logic
clearer than trying to think about high bits and low bits.

gcc/ChangeLog:

	PR target/105981
	* config/arm/arm.cc (gen_cpymem_ldrd_strd): Rename low_reg and hi_reg
	to first_reg and second_reg respectively.  Initialize them correctly
	when generating big-endian code.