src/privsep-linux.c: add support for arc (#28)

src/privsep-linux.c: add support for arc (#28)

Fix the following build failure:

privsep-linux.c:206:4: error: #error "Platform does not support seccomp filter yet"
 #  error "Platform does not support seccomp filter yet"
    ^~~~~
In file included from privsep-linux.c:36:
privsep-linux.c:213:38: error: 'SECCOMP_AUDIT_ARCH' undeclared here (not in a function); did you mean 'SECCOMP_ALLOW_ARG'?
  BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
                                      ^~~~~~~~~~~~~~~~~~

It should be noted that AUDIT_ARCH_{ARCOMPACT,ARCV2} is only defined
since kernel 5.2 and
https://github.com/torvalds/linux/commit/67f2a8a29311841ba6ab9b0e2d1b8f1e9978cd84

Detection of arc compact and arc v2 have been "copy/pasted" from
https://github.com/wbx-github/uclibc-ng/commit/afab56958f1cbb47b831ee3ebff231dfbae74af2

Fixes:
 - http://autobuild.buildroot.org/results/d29083700a80dd647621eed06faeeae03f0587d3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>