xcoff dynamic symbol string sanity

xcoff dynamic symbol string sanity

Sanity check symbol string table offsets, and tidy structs.  "long"
isn't a good choice for _l_zeroes and _l_offset since it can be 64
bits which blows out the size of the symbol struct unnecessarily.
Also, all of the sizes in internal_ldsym need only be 32 bits, but I
made them size_t because I didn't want to audit all expressions using
them for overflow.

bfd/
	* xcofflink.c (_bfd_xcoff_canonicalize_dynamic_symtab): Sanity
	check symbol _l_offset.
	(xcoff_link_add_dynamic_symbols),
	(xcoff_link_check_dynamic_ar_symbols): Likewise.
include/
	* coff/xcoff.h (struct internal_ldhdr): Tidy types.
	(struct internal_ldsym): Use uint32_t for _l_zeroes and _l_offset.