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: c92786e ("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>

Author: jason77-wang

tools/testing/selftests/rseq/Makefile

@@ -5,9 +5,13 @@ CLANG_FLAGS += -no-integrated-as
 endif
 
 top_srcdir = ../../../..
+include $(top_srcdir)/scripts/subarch.include
+ARCH ?= $(SUBARCH)
+LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
 
 CFLAGS += -O2 -Wall -g -I./ $(KHDR_INCLUDES) -L$(OUTPUT) -Wl,-rpath=./ \
-	  $(CLANG_FLAGS) -I$(top_srcdir)/tools/include
+	  $(CLANG_FLAGS) -I$(top_srcdir)/tools/include \
+	  -I$(LINUX_TOOL_ARCH_INCLUDE)
 LDLIBS += -lpthread -ldl
 
 # Own dependencies because we only want to build against 1st prerequisite, but