build.sh: optionally reduce compilation time (build gcc1 w/o optimization)

fixes #13
Updating to musl 0.9.15.
defs.sh: make it possible to override MUSL_GIT_REPO
Updated GCC patch to 48d7dbc22769.
Updating Linux headers to 3.12.6.
Use mmu_defconfig on microblaze.
A few minor grammar fixups.
Support GCC_STAGE1_NOOPT=0 as equivalent to unset._
Merge.
Preliminary x32 support.
Fix sed invocation to be compatible with Darwin sed(1)
Adding aarch64 patch from Kevin Bortis.
Updating gcc patches to musl-gcc-patches 7422b0636acd .
musl 1.0.0
Updated musl-gcc-patches to 840cc2b16514 for GCC 4.9.0.
GCC 4.8.3, musl 1.1.1.
use patched kernel-headers package from sabotage linux

this has several advantages:
- download 700 KB instead of 70MB
- much faster to unpack and "build"
- headers are patched for userspace so you don't get clashes with
  struct ethhdr and similar things.
  this is necessary to build busybox and other low-level linux utils.

only disadvantage:
- does not support new archs out-of-the-box, so when doing a port,
  one has to fallback to the vanilla linux tarball.

to achieve this in a non-ugly way, fetch-extract routine was changed
to allow usage with a single URL. it extracts the required path
components automatically.
LINUX_HEADERS_VERSION was removed as the URL is the only place using it.

closes #20
build.sh: explicitly quit with success when reaching the end

otherwise the $? variable will contain and report the status of the
last executed command.
for example building "sh4" returned error due to this last commmand:
+ [ sh4-linux-musl-strip != sh4-musl-linux-strip -a ! -e sh4-musl-linux-strip ]
make it possible to use binutils tarball which is not from gnu.org (e.g. snapshots)
make it possible to create a non-sysroot toolchain

by setting WITH_SYSROOT=0 in config.sh.

unaware of any advantages of the --sysroot switch to a crosscompiler,
i'm at least aware of one big disavantage: if sysroot was used,
the full path to libstdc++.so will end up in .la files;
and if such a .la file is referenced from a package built with
--with-sysroot=/PATH/TO/ROOTFS with the intention to make libtool
search there for libs, the absolute libstdc++.la filename will
end up with /PATH/TO/ROOTFS/PATH/TO/MUSL-CROSS/lib/libstdc++.la -
which is of course not existing.