buildroot

gdb: introduce BR2_PACKAGE_GDB_ARCH_SUPPORTS

Before adding more architecture dependencies to gdb, let's introduce a
BR2_PACKAGE_GDB_ARCH_SUPPORTS, to avoid duplicating this list of
dependencies between the Config.in comment and the main Config.in
option.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
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Changes since v2:
 - None
Changes since v1:
 - None