orcc: program-c: fix 64-bit parameter loading (loadpq) on big-endian systems

When passing 64-bit parameters through OrcExecutor, we
have to split them up into two 32-bit parameters for
backwards compatibility reasons. When generating C code,
make sure that we split up 64-bit parameters in the same
way as loadpq will read them back later. The lower 32 bits
should end up in params[ORC_VAR_D1+i] and the higher bits
should end up in params[ORC_VAR_T1+i]. The way it was done
so far, the higher/lower bits ended up swapped on big endian
systems, and then got deserialised in swapped order by loadpq.
This resulted in bogus parameters being used.

In particular, this broke the gstreamer volume element and
its unit tests on big endian systems when handling samples
in F64 format (i.e. doubles).

https://bugzilla.gnome.org/show_bug.cgi?id=739354
Preserve NEON/VFP registers across subroutines according to ARM PCS (5.1.2.1)

https://bugzilla.gnome.org/show_bug.cgi?id=727464
Fix high-order bit of first register in VPUSH/VPOP generation

CID 1256553
CID 1256554

https://bugzilla.gnome.org/show_bug.cgi?id=727464
orcc: allow setting custom backup function

Add a new .backup keyword that instructs the orc compiler to call our
custom backup function instead of generating one. This is interesting if
the generated backup function is slower than a plain C implementation.