toolchain: Add gcc patch for thumb2 targets

This adds proper thumb2 multilib support. The patch is taken from
https://github.com/prattmic/arm-cortex-m4-hardfloat-toolchain/blob/master/patches/gcc-multilib.patch

You need to use this or a similar multilib setup in order to get softfloat
working properly with newlib. The default gcc multilib setup for arm-none-eabi
will generate ARM moode instructions in the softfloat functions instead of Thumb2 instructions.

See also: https://github.com/prattmic/arm-cortex-m4-hardfloat-toolchain

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>