selftests/rseq: Fix a building error for riscv arch

selftests/rseq: Fix a building error for riscv arch

RISC-V rseq selftests include asm/fence.h from tools/arch/riscv,
but the rseq Makefile only adds tools/include in the CFLAGS, this
results in the building failure both for native and cross build:

    In file included from rseq.h:131,
                     from rseq.c:37:
    rseq-riscv.h:11:10: fatal error: asm/fence.h: No such file or directory

To fix it, add the matching tools/arch/$(ARCH)/include path in the
CFLAGS and derive ARCH from SUBARCH for standalone native builds where
ARCH is not set.

Fixes: c92786e179e0 ("KVM: riscv: selftests: Use the existing RISCV_FENCE macro in `rseq-riscv.h`")
Cc: stable@vger.kernel.org
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://patch.msgid.link/20260707082348.36896-1-hui.wang@canonical.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>