gcc: xtensa: fix PR target/105879

gcc: xtensa: fix PR target/105879

split_double operates with the 'word that comes first in memory in the
target' terminology, while gen_lowpart operates with the 'value
representing some low-order bits of X' terminology. They are not
equivalent and must be dealt with differently on little- and big-endian
targets.

gcc/
	PR target/105879
	* config/xtensa/xtensa.md (movdi): Rename 'first' and 'second'
	to 'lowpart' and 'highpart' so that they match 'gen_lowpart' and
	'gen_highpart' bitwise semantics and fix order of highpart and
	lowpart depending on target endianness.