gdb: handle case where type alignment is unknown

gdb: handle case where type alignment is unknown

It was spotted that if type_align returned 0 then it was possible to
trigger a divide by zero exception within GDB.  It turns out this will
only happen in an edge case where GDB is unable to figure out the
alignment of a field within a structure.

The attached test generates some non-standard, probably broken, DWARF,
that triggers this condition, and then fixes this issue by throwing an
exception when this case occurs.

gdb/ChangeLog:

	PR gdb/27847
	* amd64-tdep.c (amd64_has_unaligned_fields): Move call to
	type_align, and spot case where the alignment is unknown.

gdb/testsuite/ChangeLog:

	PR gdb/27847
	* gdb.dwarf2/dw2-weird-type-len.c: New file.
	* gdb.dwarf2/dw2-weird-type-len.exp: New file.