Determine path to libgcc.a automatically

[Was](https://travis-ci.org/hackndev/zinc/jobs/28587341#L116): `/usr/lib/gcc/arm-none-eabi/4.8.4/armv7-m/libgcc.a`
[Now](https://travis-ci.org/errordeveloper/zinc/jobs/28588446#L114): `/usr/bin/../lib/gcc/arm-none-eabi/4.8.4/armv7-m/libgcc.a`