Some platforms such as x86 and amd64 have efficient unaligned access.

Some platforms such as x86 and amd64 have efficient unaligned access.
On these platforms, implement read_/write_<type> by doing a direct
access, rather than calling a function that will read or write
'byte per byte'.

For platforms that do not have efficient unaligned access,
or that do not support at all unaligned access, call function
  readUAS_/writeUAS_<type> that works as before.

Currently, direct acecss is activated only for x86 and amd64.
Unclear what other platforms support (efficiently) unaligned access.

On unwind intensive code (such as perf/memrw on amd64), this patch
gives up to 5% improvement.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15290