common/BackTrace: demangle on FreeBSD also

common/BackTrace: demangle on FreeBSD also

the output on FreeBSD/clang looks like:

1: 0x44bfb3 <_Z3foov+0x413> at /usr/srcs/Ceph/work/ceph/build/bin/unittest_back_trace
2: 0x44c23e <_ZN20BackTrace_Basic_Test8TestBodyEv+0x1e> at /usr/srcs/Ceph/work/ceph/build/bin/unittest_back_trace
3: 0x4d068a <_ZN7testing8internal38HandleSehExceptionsInMethodIfSupportedINS_4TestEvEET0_PT_MS4_FS3_vEPKc+0x7a> at /usr/srcs/Ceph/work/ceph/build/bin/unittest_back_trace
4: 0x4b5977 <_ZN7testing8internal35HandleExceptionsInMethodIfSupportedINS_4TestEvEET0_PT_MS4_FS3_vEPKc+0x77> at /usr/srcs/Ceph/work/ceph/build/bin/unittest_back_trace
...

and update the test accordingly, as FreeBSD/clang uses '<>' to enclose
the mangled function and offset.

also, only demangle the C++ mangled names. those names always start with
"_Z". on FreeBSD, after demangling, "main" is turned into "unsigned
long", which does not make sense.

Signed-off-by: Kefu Chai <kchai@redhat.com>