don't enable prefer_lib_subdir by default in GCC easyblock

[boegel]

This was enabled by default in #1030 with the intention of making sure that /lib subdirectories of the dependencies that were installed with EasyBuild get preference over /usr/lib64.

However, it has the side-effect of also preferring /usr/lib over /usr/lib64, which leads to problem like these:

• GCC (see also disable patching of GCC to make it prefer /lib over /lib64 to avoid breaking the build easybuild-easyconfigs#3801):

/usr/lib/crti.o: could not read symbols: File in wrong format
collect2: ld returned 1 exit status

with:

$ file /usr/lib/crti.o
/usr/lib/crti.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped

• MPICH, MVAPICH & other MPI libraries, if 32-bit BLCR is installed system-wide:

/usr/lib/libcr.so: could not read symbols: File in wrong format

with

$ file /usr/lib/libcr.so
/usr/lib/libcr.so: symbolic link to `libcr.so.0.5.5'
$ file /usr/lib/libcr.so.0.5.5
/usr/lib/libcr.so.0.5.5: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped

I think we probably could enable this by default for sufficiently recent versions of binutils (which are potentially smart enough to skip incompatible libraries), but I'm not 100% sure, and I don't have time to figure this out right now.

cc @sebth, @geimer (you mentioned something about side effects in #921... ;-))