gdb/arm: move fetch of arm_gdbarch_tdep to a more inner scope

gdb/arm: move fetch of arm_gdbarch_tdep to a more inner scope

This is a small refactor to resolve an issue before it becomes a
problem in a later commit.

Move the fetching of an arm_gdbarch_tdep into a more inner scope
within two functions in arm-tdep.c.

The problem with the current code is that the functions in question
are used as the callbacks for two set/show parameters.  These set/show
parameters are available no matter the current architecture, but are
really about controlling an ARM architecture specific setting.  And
so, if I build GDB for all targets on an x86-64/GNU-Linux system, I
can still do this:

  (gdb) show arm fpu
  (gdb) show arm abi

After these calls we end up in show_fp_model and arm_show_abi
respectively, where we unconditionally do this:

  arm_gdbarch_tdep *tdep
    = (arm_gdbarch_tdep *) gdbarch_tdep (target_gdbarch ());

However, the gdbarch_tdep() result will only be a arm_gdbarch_tdep if
the current architecture is ARM, otherwise the result will actually be
of some other type.

This isn't actually a problem, as in both cases the use of tdep is
guarded by a later check that the gdbarch architecture is
bfd_arch_arm.

This commit just moves the call to gdbarch_tdep() after the
architecture check.

In a later commit gdbarch_tdep() will be able to spot when we are
casting the result to the wrong type, and this function will trigger
assertion failures if things are not fixed.

There should be not user visible changes after this commit.