RISC-V: Add address printer tests with ADDIW

RISC-V: Add address printer tests with ADDIW

Address sequences involving ADDIW/C.ADDIW instructions require special
handling to sign-extend lower 32-bits of the original result.

This commit tests whether this sign-extension works.

gas/ChangeLog:

	* testsuite/gas/riscv/dis-addr-addiw.s: New to test the address
	computation with sign extension as used in ADDIW/C.ADDIW.
	* testsuite/gas/riscv/dis-addr-addiw-a.d: Test PC sign bit 0.
	* testsuite/gas/riscv/dis-addr-addiw-b.d: Test PC sign bit 1.

gas/ChangeLog:

	* testsuite/gas/riscv/dis-addr-addiw-a.d: New test.
	* testsuite/gas/riscv/dis-addr-addiw-b.d: New test.
	* testsuite/gas/riscv/dis-addr-addiw.s: New test.