iconv: Remove _STRING_ARCH_unaligned usage

Use put/get macros __builtin_bswap32 instead.  It allows to remove
the unaligned routines, the compiler will generate unaligned access
if the ABI allows it.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>