Fix default libdir for Debian/Ubuntu

The distribution provided architecture specific RPM macro files
for x86_64 and other architectures on Debian/Ubuntu specify the
wrong default libdir install location. When building deb packages
override _lib with the correct location.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7083
Closes #7101