Centralize arch detection in a public header

Centralize arch detection in a public header

Add a public header with centralized compiler arch detection and arch
specific parameters. Namespace everything under 'LTTNG_UST_ARCH_'.

Move "LTTNG_UST_HAVE_EFFICIENT_UNALIGNED_ACCESS" from a configure time
define to a static one base on compiler arch detection.

This will simplify the build system and remove the possibility of
accidently mixing aligned and unaligned access in a multiarch
environment with shared header files.

Change-Id: Ic977d13f031ad070bea4ff5d7b2b8079843e0c26
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>