libatlas: use uname -m to detect arch.

output of uname -p is "unknown" for soem processors, leading to use 32bits on 64bits machine.
version up : groonga 4.0.7 and mroonga 4.07.
version up : wget 1.16.
version up : gettext 0.19.3.
version up : LibreOffice 4.3.3.2.