bpf: fix cross build failure on Debian

For compiling bpf code, the system include directory needs to be
constructed. On Debian-like systems, this requires passing a multiarch
directory. Since clang's -dump-machine prints something other that the
multiarch triplet, gcc was interrogated earlier, but that also yields a
wrong result for cross compilation and was thus skipped resulting in
clang not finding asm/types.h.

Rather than, -dump-machine we should ask for -print-multiarch (which
rarely differs). Whenever gcc is in use, this is right (even for cross
building). Since clang does not support -print-multiarch and its
-dump-machine never matches Debian's multiarch, we resort to asking gcc
when building natively. For cross builds using clang, we are out of
luck.

meson.build

@@ -1823,8 +1823,16 @@ if conf.get('BPF_FRAMEWORK') == 1
 
         bpf_o_unstripped_cmd += ['-I.']
 
-        if not meson.is_cross_build()
-                target_triplet_cmd = run_command('gcc', '-dumpmachine', check: false)
+        if cc.get_id() == 'gcc' or meson.is_cross_build()
+                if cc.get_id() != 'gcc'
+                        warning('Cross compiler is not gcc. Guessing the target triplet for bpf likely fails.')
+                endif
+                target_triplet_cmd = run_command(cc.cmd_array(), '-print-multiarch', check: false)
+        else
+                # clang does not support -print-multiarch (D133170) and its -dump-machine
+                # does not match multiarch. Query gcc instead.
+                target_triplet_cmd = run_command('gcc', '-print-multiarch', check: false)
+        endif
         if target_triplet_cmd.returncode() == 0
                 target_triplet = target_triplet_cmd.stdout().strip()
                 bpf_o_unstripped_cmd += [
@@ -1832,7 +1840,6 @@ if conf.get('BPF_FRAMEWORK') == 1
                         '/usr/include/@0@'.format(target_triplet)
                 ]
         endif
-        endif
 
         bpf_o_unstripped_cmd += [
                 '-idirafter',