[tools-build.py]Enforce supported toolchains

[theotherjimmy]

by skipping unsupported combinations

before:

$ python tools/build.py -t IAR -m NRF51_DK --source . --build build
[SNIP]
Compile: us_ticker.c
  #error Compiler not supported.
   ^
"Z:\home\jimbri01\src\mbedmicro\mbed\hal\targets\hal\TARGET_NORDIC\TARGET_MCU_NRF51822\us_ticker.c",382  Fatal error[Pe035]: #error directive: Compiler not supported.
Fatal error detected, aborting.

  #error Compiler not supported.
   ^
"Z:\home\jimbri01\src\mbedmicro\mbed\hal\targets\hal\TARGET_NORDIC\TARGET_MCU_NRF51822\us_ticker.c",382  Fatal error[Pe035]: #error directive: Compiler not supported.
Fatal error detected, aborting.


Completed in: (8.96)s

Build failures:
  * IAR::NRF51_DK

after:

$ python tools/build.py -t IAR -m NRF51_DK --source . --build build
IAR::NRF51_DK skipped: toolchain not supported

Completed in: (0.00)s

Build skipped:
  * IAR::NRF51_DK

[theotherjimmy]

cc @bridadan @screamerbg

[bridadan]

@theotherjimmy Should we merge this guy first so we can test with it before merging this PR?

[theotherjimmy]

Yes, that is a good idea.

[screamerbg]

LGTM