testsuite:: Fix pr39986.c testcase for AArch64

The testcase in c-c++-common/dfp/pr39986.c detects if DFP constants
are correctly emitted in the assembly.  However, AArch64 uses .word
instead of the expected .long directive. With this patch, we now
accept both.

2022-03-31  Christophe Lyon <christophe.lyon@arm.com>

	gcc/testsuite/
	* c-c++-common/dfp/pr39986.c: Accept .word directive.