fails to build with libseccomp-dev installed on arm64

13:21:06 ../tor/src/common/sandbox.c: In function 'sigsys_debugging':
13:21:06 ../tor/src/common/sandbox.c:1812:35: error: 'mcontext_t {aka struct sigcontext}' has no member named 'M_SYSCALL'
13:21:06    syscall = (int) ctx->uc_mcontext.M_SYSCALL;
13:21:06                                    ^
13:21:13 Makefile:5615: recipe for target 'src/common/sandbox.o' failed
13:21:13 make[1]: *** [src/common/sandbox.o] Error 1

Note that on arm64, __arm__ is not defined.

cf. https://wiki.debian.org/ArchitectureSpecificsMemo