Add support for vanilla linux on arm.
auto merge of #8220 : luqmana/rust/arm-linux, r=cmr

Update the arm linux support some more. We had a previous patch for the RasberryPi. This adds a new target `arm-unknown-linux-gnueabi` for more general arm linux support.

Build/Host machine: x86_64 Debian testing (jessie) with the `gcc-4.4-arm-linux-gnueabi` package

Tested on targets:

- TS-7800 Feroceon (ARMv5TEJ) running Debian 7.0 wheezy
- Beaglebone black (ARMv7) running Angstrom GNU/Linux v2012.12
   - rustc flags:  `--target=arm-unknown-linux-gnueabi --linker=arm-linux-gnueabi-gcc`

- Samsung Galaxy S II (to make sure android still works)
   - rustc flags:  `--target=arm-linux-androideabi --android-cross-path=[path to standalone toolchain]`

Since not all arm devices (i.e. afaik anything older than armv6 like the ts-7800 i tested on) supported getting the tls address via the `mrc` instruction, I made it also try via the magic address the kernel maps into the address space (0xFFFF0FF0). One or the other should work (and on android it seems like both work).

Also fixes a bug where rustc would always try to invoke the android assembler for any kind of arm target.