Tools: Use Distcc when it's configured

[theotherjimmy]

Description

Distcc is a distributed C/C++ compiler.
It can easily take compile jobs and speed them up by distributing
the individual jobs amound many computers. Distcc is configured with the
DISTCC_POTENTIAL_HOSTS environment variable. This patch configures GCC_ARM
to use the Distcc when that environment variable is set.

@bremoran

Pull request type

[ ] Fix
[ ] Refactor
[ ] New target
[x] Feature
[ ] Breaking change

[cmonr]

Neat!

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2858
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7846/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2488
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7846/

[bremoran]

LGTM!

[mbed-ci]

Test : SUCCESS

Build number : 2620
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7846/2620