update description of cross compiler requirements
update description of cross compiler requirements

Moderately recent versions of ubuntu (i.e. all supported versions
except 16.04LTS) come with arm-none-eabi-gcc version 6.3.1 (which is
the same version as used to compile newlib/cortex-m/libc.a) or later,
so there is no longer a  need to install it from a ppa.

tock/libtock-c,tock/libtock-c,tock/libtock-c