mmix-mmo set SEC_DATA for .data section

With the bfd_decode_symclass change, ld SIZEOF test fails due to
data_end being reported as type '?'.

bfd/
	PR 24511
	* mmo.c (mmo_scan): Set SEC_DATA for .data.
ld/
	PR 24511
	* testsuite/ld-mmix/b-fixo2.d: Adjust for .data change.
	* testsuite/ld-mmix/sec-2.d: Likewise.
	* testsuite/ld-mmix/sec-3.d: Likewise.
	* testsuite/ld-mmix/sec-4.d: Likewise.
	* testsuite/ld-mmix/spec802.d: Likewise.
	* testsuite/ld-mmix/spec803.d: Likewise.
	* testsuite/ld-mmix/spec804.d: Likewise.
	* testsuite/ld-mmix/spec805.d: Likewise.
	* testsuite/ld-mmix/spec806.d: Likewise.
	* testsuite/ld-mmix/spec807.d: Likewise.
	* testsuite/ld-mmix/spec808.d: Likewise.