aarch64: Fix test_dfp_17.c for big-endian [PR 107604]

gcc.target/aarch64/aapcs64/test_dfp_17.c has been failing on
big-endian, because the _Decimal32 on-stack argument is not padded in
the same direction depending on endianness.

This patch fixes the testcase so that it expects the argument in the
right stack location, similarly to what other tests do in the same
directory.

gcc/testsuite/ChangeLog:

	PR target/107604
	* gcc.target/aarch64/aapcs64/test_dfp_17.c: Fix for big-endian.