[gdb] Name arch selftests more clearly

[gdb] Name arch selftests more clearly

When running some all archs selftest I get:
...
$ gdb -q -batch -ex "maint selftest unpack_field_as_long"
Running selftest unpack_field_as_long::A6.
...

By now I know that A6 is an arc architecture, but for others that's less
clear.

Fix this by using unpack_field_as_long::arc::A6 instead.

This then introduces redundant names like arm::arm, so try to avoid those,
though I'm not entirely convinced that that's worth the trouble.

This introduces the following new names:
...
+Running selftest unpack_field_as_long::am33_2::am33-2.
+Running selftest unpack_field_as_long::arc::A6.
+Running selftest unpack_field_as_long::arc::A7.
+Running selftest unpack_field_as_long::arc::EM.
+Running selftest unpack_field_as_long::arc::HS.
+Running selftest unpack_field_as_long::arm::ep9312.
+Running selftest unpack_field_as_long::arm::iwmmxt.
+Running selftest unpack_field_as_long::arm::iwmmxt2.
+Running selftest unpack_field_as_long::arm::xscale.
+Running selftest unpack_field_as_long::bpf::xbpf.
+Running selftest unpack_field_as_long::frv::fr400.
+Running selftest unpack_field_as_long::frv::fr450.
+Running selftest unpack_field_as_long::frv::fr500.
+Running selftest unpack_field_as_long::frv::fr550.
+Running selftest unpack_field_as_long::frv::simple.
+Running selftest unpack_field_as_long::frv::tomcat.
+Running selftest unpack_field_as_long::iq2000::iq10.
+Running selftest unpack_field_as_long::m32c::m16c.
+Running selftest unpack_field_as_long::mep::c5.
+Running selftest unpack_field_as_long::mep::h1.
+Running selftest unpack_field_as_long::nds32::n1.
+Running selftest unpack_field_as_long::nds32::n1h.
+Running selftest unpack_field_as_long::nds32::n1h_v2.
+Running selftest unpack_field_as_long::nds32::n1h_v3.
+Running selftest unpack_field_as_long::nds32::n1h_v3m.
+Running selftest unpack_field_as_long::z80::ez80-adl.
+Running selftest unpack_field_as_long::z80::ez80-z80.
+Running selftest unpack_field_as_long::z80::gbz80.
+Running selftest unpack_field_as_long::z80::r800.
+Running selftest unpack_field_as_long::z80::z180.
...

Tested on x86_64-linux.