gdb: add supports_arch_info callback to gdbarch_register

gdb: add supports_arch_info callback to gdbarch_register

In the ROCm GDB port, there are some amdgcn architectures known by BFD
that we don't actually support in GDB.  We don't want
gdbarch_printable_names to return these architectures.

gdbarch_printable_names is used for a few things:

 - completion of the "set architecture" command
 - the gdb.architecture_names function in Python
 - foreach-arch selftests

Add an optional callback to gdbarch_register that is a predicate
indicating whether the gdbarch supports the given bfd_arch_info.  by
default, it is nullptr, meaning that the gdbarch accepts all "mach"s for
that architecture known by BFD.

Change-Id: I712f94351b0b34ed1f42e5cf7fc7ba051315d860
Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>