Prevent compiling with unsupported compilers

[theotherjimmy]

Description

Before this change it is possible to compile with a compiler that a
device port does not support. Afterwards it will not be.

It is possible to override the supported toolchains in the application
configuration and compile with the unsupported toolchain after this change.

Resolves #5109

Pull request type

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

[bridadan]

Looks good, with a few follow up questions/potential tasks:

1. Might be a good idea to add a test for target_supports_toolchain, specifically for the ARM case.
2. In the NotSupportedException, do we want to also state that "It is possible to override the supported toolchains in the application configuration and compile with the unsupported toolchain"?

[0xc0170]

Please review travis CI, failures related

[theotherjimmy]

All of the tests needed to support gcc_arm.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1666
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6540/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 1461
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6540/1461

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1300
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6540/

[cmonr]

Rerunning since last build is stale, but will be first in queue!

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1827
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6540/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1471
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6540/

[mbed-ci]

Test : FAILURE

Build number : 1633
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6540/1633

[0xc0170]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 1640
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6540/1640