Reduce build disk usage about crosstool-ng-old HOT 4 CLOSED

Fixed by #9

from crosstool-ng-old.

Re-opening, since further optimisation is necessary.

from crosstool-ng-old.

In spite of having implemented a step-level build directory clean-up mechanism, the build still goes over the disk space limit during the final gcc (host-to-target) build process -- implying that this process alone is enough to go over the limit.

A further analysis revealed that the libstdc++-v3 multilib build, which is executed as part of the final gcc build process, is taking up a significant amount of disk space:

471M    arc600
471M    arc600_mul32x16
471M    arc600_mul64
471M    arc600_norm
471M    arc601
471M    arc601_mul32x16
471M    arc601_mul64
471M    arc601_norm
471M    arc700
470M    arcem
469M    archs
470M    em
470M    em4
470M    em4_dmips
470M    em4_fpuda
470M    em4_fpus
470M    em_mini
469M    hs
469M    hs34
469M    hs38
469M    hs38_linux
469M    hs4x
469M    hs4xd
5.5M    libgcc
465M    libstdc++-v3
471M    nps400
470M    quarkse_em
12G     .

Since this process is invoked from within the GCC build system, we need to add GCC patches to delete the build files after each libstdc++-v3 multilib build step.

from crosstool-ng-old.

Since the current GCC build process is designed such that all multilib variants are built first (all target) and then installed (install target), we need to do either of the following:

1. Only preserve the bare minimum (i.e. leave .a files while nuking .gch files which take up most of the disk space) necessary to make library installation successful.
2. Rework the build process to build-install-prune each multilib variant.

EDIT:
.gch files are the major offenders:

@ /.build/arc-zephyr-elf/build/build-cc-gcc-final/arc-zephyr-elf/em4/libstdc++-v3/include/arc-zephyr-elf/bits
106M    extc++.h.gch
139M    stdc++.h.gch
77M     stdtr1c++.h.gch

After removing the gch files, each multilib variant takes up about 150MB. In case of ARC, there are 26 multilib variants, and this means the disk usage will be approx. 4GB, which is reasonable.

from crosstool-ng-old.