bpf: fix broken BPF selftest build

At least on x86_64, the kernel's BPF selftests seemed to have stopped
to build due to 618e165b2a8e ("selftests/bpf: sync kernel headers and
introduce arch support in Makefile"):

[...]
In file included from test_verifier.c:29:0:
../../../include/uapi/linux/bpf_perf_event.h:11:32:
fatal error: asm/bpf_perf_event.h: No such file or directory
#include <asm/bpf_perf_event.h>
^
compilation terminated.
[...]

While pulling in tools/arch/*/include/uapi/asm/bpf_perf_event.h seems
to work fine, there's no automated fall-back logic right now that would
do the same out of tools/include/uapi/asm-generic/bpf_perf_event.h. The
usual convention today is to add a include/[uapi/]asm/ equivalent that
would pull in the correct arch header or generic one as fall-back, all
ifdef'ed based on compiler target definition. It's similarly done also
in other cases such as tools/include/asm/barrier.h, thus adapt the same
here.

Fixes: 618e165b2a8e ("selftests/bpf: sync kernel headers and introduce arch support in Makefile")
Signed-off-by: Daniel Borkmann <[e-mail]>
Cc: Hendrik Brueckner <[e-mail]>
Cc: Arnaldo Carvalho de Melo <[e-mail]>
Acked-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/include/uapi/asm/bpf_perf_event.h

···
+#if defined(__aarch64__)
+#include "../../arch/arm64/include/uapi/asm/bpf_perf_event.h"
+#elif defined(__s390__)
+#include "../../arch/s390/include/uapi/asm/bpf_perf_event.h"
+#else
+#include <uapi/asm-generic/bpf_perf_event.h>
+#endif

tools/testing/selftests/bpf/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
-ifeq ($(srctree),)
-srctree := $(patsubst %/,%,$(dir $(CURDIR)))
-srctree := $(patsubst %/,%,$(dir $(srctree)))
-srctree := $(patsubst %/,%,$(dir $(srctree)))
-srctree := $(patsubst %/,%,$(dir $(srctree)))
-endif
-include $(srctree)/tools/scripts/Makefile.arch
-
-$(call detected_var,SRCARCH)
-
 LIBDIR := ../../../lib
 BPFDIR := $(LIBDIR)/bpf
 APIDIR := ../../../include/uapi
-ASMDIR:= ../../../arch/$(ARCH)/include/uapi
 GENDIR := ../../../../include/generated
 GENHDR := $(GENDIR)/autoconf.h
 
···
   GENFLAGS := -DHAVE_GENHDR
 endif
 
-CFLAGS += -Wall -O2 -I$(APIDIR) -I$(ASMDIR) -I$(LIBDIR) -I$(GENDIR) $(GENFLAGS) -I../../../include
+CFLAGS += -Wall -O2 -I$(APIDIR) -I$(LIBDIR) -I$(GENDIR) $(GENFLAGS) -I../../../include
 LDLIBS += -lcap -lelf
 
 TEST_GEN_PROGS = test_verifier test_tag test_maps test_lru_map test_lpm_map test_progs \