Fix and simplify library folder for unix

Using QMAKE_HOST.arch is not the best of all possibilities to do so:
<pre>
cd src/plugins/ && ( test -e Makefile || /usr/lib/i386-linux-gnu/qt5/bin/qmake /tmp/buildd/qupzilla-1.6.6.siduction.8/src/plugins/plugins.pro -nocache QMAKE_CFLAGS_RELEASE=-g\ -O2\ -fstack-protector\ --param=ssp-buffer-size=4\ -Wformat\ -Werror=format-security\ -D_FORTIFY_SOURCE=2 QMAKE_CFLAGS_DEBUG=-g\ -O2\ -fstack-protector\ --param=ssp-buffer-size=4\ -Wformat\ -Werror=format-security\ -D_FORTIFY_SOURCE=2 QMAKE_CXXFLAGS_RELEASE=-g\ -O2\ -fstack-protector\ --param=ssp-buffer-size=4\ -Wformat\ -Werror=format-security\ -D_FORTIFY_SOURCE=2 QMAKE_CXXFLAGS_DEBUG=-g\ -O2\ -fstack-protector\ --param=ssp-buffer-size=4\ -Wformat\ -Werror=format-security\ -D_FORTIFY_SOURCE=2 QMAKE_LFLAGS_RELEASE=-Wl,-z,relro QMAKE_LFLAGS_DEBUG=-Wl,-z,relro QMAKE_STRIP=: PREFIX=/usr -o Makefile ) && make -f Makefile
Project MESSAGE: QMAKE_HOST.arch: x86_64
Project MESSAGE: QT_ARCH: i386
Project MESSAGE: QUPZILLA_PREFIX:
Project MESSAGE: library folder /usr/lib/i386-linux-gnu
</pre>
(Build in a i386-build-chroot on a amd64 host)