PR 34053 buffer overflow in xcoff_link_add_symbols

This patch adds two sanity checks with error reporting in
xcoff_link_add_symbols before reading symbol aux entries, add extends
assertions in later functions.  A whole lot of unnecessary casts are
also tidied.

	PR 34053
	* xcofflink.c: Remove unnecessary casts throughout.
	(xcoff_link_add_symbols): Sanity check aux entries are within
	symbol buffer.
	(bfd_xcoff_build_dynamic_sections): Assert the above is true.
	(xcoff_link_input_bfd): Likewise.