gcc-initial, gcc-intermediate, gcc-final: optimize extraction

Several sub-directories of the gcc code base are in fact not needed
for the Buildroot build: libjava/, libgo/ and gcc/testsuite/ being the
biggest ones. Avoiding their extraction saves quite a bit of disk
space, and compensates a bit the fact that we now extract three times
the gcc source code.

This requires changing the 100-uclibc-conf.patch to no longer patch
files from the libjava/ directory, since this directory is no longer
extracted.

[Peter: add comment about why this is done]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>