gdb/selftest-arch: Make register_test_foreach_arch generate arch tests lazily

gdb/selftest-arch: Make register_test_foreach_arch generate arch tests lazily

The register_test_foreach_arch is used to instantiate a given selftest
for all architectures supported by GDB.  It is used in many _initialize_*
functions (under initialize_all_files, called by gdb_init).

Because the call is done during GDB's initialization, and because there
is no guaranty about the order in which all the _initialize_* functions
are executed, when register_test_foreach_arch is called, GDB is not
fully initialized.  Specifically, when a particular initialize function
is executed, only the architectures registered at that point are listed
by gdbarch_printable_names.

As a consequence, the list of selftest effectively executed depends on
the order the _initialize_* functions are called.  This can be observed
with the following:

    $ ./gdb/gdb \
        -data-directory ./gdb/data-directory \
        -quiet -batch -ex "maint selftest" 2>&1 \
        | grep -E "Ran [0-9]+ unit tests"
    Ran 145 unit tests, 0 failed
    $ GDB_REVERSE_INIT_FUNCTIONS=1 ./gdb/gdb \
        -data-directory ./gdb/data-directory \
        -quiet -batch -ex "maint selftest" 2>&1 \
        | grep -E "Ran [0-9]+ unit tests"
    Ran 82 unit tests, 0 failed

To fix this, make register_test_foreach_arch register a lazy selftest
generator.  This way when the test generator is eventually executed, all
architectures are registered and we do not have a dependency on the
order the initialize functions are executed in.

Tested on x86_64-linux

Change-Id: I88eefebf7d372ad672f42d3a103e89354bc8a925