Linux 64 bit: added x86_64 build support (including build for Firefox 19)

Get architecture with uname -m and build libwebcl64.so if x86_64 detected. 
Updated chrome.manifest accordingly.

xpi built using: 
xulrunner-19.0.1.en-US.linux-x86_64.sdk
gcc version 4.7.2 (Ubuntu/Linaro 4.7.2-2ubuntu1)