gdb: Allow gdbarch to override alignment for method and member pointers

gdb: Allow gdbarch to override alignment for method and member pointers

The code in type_align (gdbtypes.c) currently hard-codes the rules for
aligning method and member pointers.  It would seem better to forward
these types through the gdbarch hook, so that an architecture could
override the alignment of these types if needed.

Only 3 architectures currently override the gdbarch alignment hook,
these are arc, i386, and nio2.

For arc and nios the alignment rules are that alignment is the minimum
of 4-bytes and the type length.  As pointers are 4-bytes on these
targets, then (assuming method and members pointers are also 4-bytes)
there should be no change to the alignment after this patch.

For i386 the gdbarch alignment hook overrides for some INT and FLOAT
types only.  For method and member pointers we align on the type size
still, so there should be no change to the alignment after this patch.

I tested this on x86-64 GNU Linux with no regressions.

gdb/ChangeLog:

	* gdbtypes.c (type_align): Allow alignment of TYPE_CODE_METHODPTR
	and TYPE_CODE_MEMBERPTR to be overridden by the gdbarch.