Buildbot: Make archive naming shorter and cleaner

It changes name to be blender-<version>-linux64.

Since CentOS is used as a base host for builds there is no real need
in specifying libc version. Is unlikely anything older could be used
anyway.

Also make bitness to be the same as windows. It is something what
users will read easier.

build_files/buildbot/slave_pack.py

@@ -160,8 +160,6 @@ def pack_linux(builder):
     blender_executable = os.path.join(builder.install_dir, 'blender')
 
     info = buildbot_utils.VersionInfo(builder)
-    blender_glibc = builder.name.split('_')[1]
-    blender_arch = 'x86_64'
 
     # Strip all unused symbols from the binaries
     print("Stripping binaries...")
@@ -172,7 +170,7 @@ def pack_linux(builder):
     buildbot_utils.call(builder.command_prefix + ['find', py_target, '-iname', '*.so', '-exec', 'strip', '-s', '{}', ';'])
 
     # Construct package name
-    platform_name = 'linux-' + blender_glibc + '-' + blender_arch
+    platform_name = 'linux64'
     package_name = get_package_name(builder, platform_name)
     package_filename = package_name + ".tar.xz"