testsuite: Add C++ unwinding tests with Decimal Floating-Point

These tests exercise exception handling with Decimal Floating-Point
type.

dfp-1.C and dfp-2.C check that thrown objects of such types are
properly caught, whether when using C++ classes (decimalXX) or via GCC
mode attributes.

dfp-saves-aarch64.C checks that such objects are properly restored,
and has to use the mode attribute trick because objects of decimalXX
class type cannot be assigned to a register variable.

2022-05-03  Christophe Lyon  <christophe.lyon@arm.com>

	gcc/testsuite/
	* g++.dg/eh/dfp-1.C: New test.
	* g++.dg/eh/dfp-2.C: New test.
	* g++.dg/eh/dfp-saves-aarch64.C: New test.