libelf linkrot

[Bothersome Bloke]

I followed the build instructions for the latest release:

```
$ wget https://developer.arm.com/-/media/Files/downloads/gnu-rm/8-2018q4/gcc-arm-none-eabi-8-2018-q4-major-src.tar.bz2
$ tar xf gcc-arm-none-eabi-8-2018-q4-major-src.tar.bz2
$ cd gcc-arm-none-eabi-8-2018-q4-major
./install-sources.sh
```

This runs for a while but dies (with no explanation)
```
+ for prereq in '$PREREQS'
+ '[' -z 'ENV_VAR_UPDATE PYTHON_WIN' -a no = yes ']'
+ eval 'prereq_pack=$LIBELF_PACK'
++ prereq_pack=libelf-0.8.13.tar.gz
+ '[' '!' -f libelf-0.8.13.tar.gz ']'
+ eval 'prereq_url=$LIBELF_URL'
++ prereq_url=http://www.mr511.de/software/libelf-0.8.13.tar.gz
+ wget -q http://www.mr511.de/software/libelf-0.8.13.tar.gz
```

The reason turns out to be: mr511.de has gone offline with no obvious redirect or mirror.

brew is (planning to) fix(ed) this by using archive.org: https://github.com/Homebrew/homebrew-core/issues/39870

I applied the same basic patch to your build scripts and was able to run `./install-sources.sh` to completion:

```
--- build-common.sh.1 2019-05-17 10:21:55.000000000 -0400
+++ build-common.sh 2019-05-16 20:05:44.000000000 -0400
@@ -299,7 +299,7 @@
 MPC_URL=ftp://ftp.gnu.org/gnu/mpc/$MPC_PACK
 ISL_URL=http://isl.gforge.inria.fr/$ISL_PACK
 EXPAT_URL=https://downloads.sourceforge.net/project/expat/expat/$EXPAT_VER/$EXPAT_PACK
-LIBELF_URL=http://www.mr511.de/software/$LIBELF_PACK
+LIBELF_URL=https://web.archive.org/web/20181111033959/http://www.mr511.de/software/$LIBELF_PACK
 LIBICONV_URL=https://ftp.gnu.org/pub/gnu/libiconv/$LIBICONV_PACK
 ZLIB_URL=http://www.zlib.net/fossils/$ZLIB_PACK
 ENV_VAR_UPDATE_URL=http://nsis.sourceforge.net/mediawiki/images/a/ad/$ENV_VAR_UPDATE_PACK
```

With this failing --- or if you were just trying to do a build on an offline machine --- the build scripts fail to unpack the main sources breaking the entire build. I have libelf installed independently via brew already---I don't need gcc to reinstall it for me It would be an improvement if the build scripts unpacked the local sources (newlib.tar.gz, gdb.tar.gz, gcc.tar.gz, binutils.tar.gz) before trying to download anything.