* Drop unused variables from build system that refer to the system
libraries.
* Drop unused lists of libgcc objects.
* Drop no longer used variables from configuration script.
* Remove no longer needed building of kernel-libgcc and -libsupc++ from
build_cross_tools_gcc4, only the boot-specific (32-bit) libs are
built for a x86_64 target.
* Explicitly disable threads and TLS support when building the cross
compiler, as the only libraries that are used by Haiku's build system
is the 32-bit libgcc and libsupc++ for the bootloader on x86_64 (and
for that neither is wanted).
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