gulp build fails if toolchain is specified by MBED_GCC_ARM_PATH

The following command fails if gcc toolchain is specified only by the MBED_GCC_ARM_PATH environment variable.
`gulp build --js ./source/main.js --target=K64F`

Error Message:
```
mbed-js-v2 user$ gulp build --js ./source/main.js --target=K64F
[14:55:21] Using gulpfile ~/mbed/mbed-js-v2-org/gulpfile.js
[14:55:21] Starting 'bundle'...
[14:55:21] Starting 'pip-requirements'...
[14:55:21] Starting 'make-build-dir'...
[14:55:21] Finished 'make-build-dir' after 601 μs
[14:55:21] Finished 'bundle' after 147 ms
[14:55:22] Finished 'pip-requirements' after 827 ms
[14:55:22] Starting 'cppify'...
[14:55:22] Starting 'generate-pins'...
[14:55:22] Finished 'cppify' after 171 ms
[14:55:24] 'generate-pins' errored after 1.67 s
[14:55:24] ChildProcessError: `python /Users/user/mbed/mbed-js-v2/mbed-js/tools/generate_pins.py K64F` failed with code 1
at ChildProcess. (/Users/user/mbed/mbed-js-v2/node_modules/child-process-promise/lib/index.js:132:23)
at emitTwo (events.js:126:13)
at ChildProcess.emit (events.js:214:7)
at maybeClose (internal/child_process.js:925:16)
at Socket.stream.socket.on (internal/child_process.js:346:11)
at emitOne (events.js:116:13)
at Socket.emit (events.js:211:7)
at Pipe._handle.close [as _onclose] (net.js:554:12)

```

To avoid the error, the gcc toolchain path should be included in the PATH environment variable.

Start with gulpfile.js and trace the generate-pins task that invokes mbed-js/tools/generate_pins.py for the K64F target. Reproduce the command with only MBED_GCC_ARM_PATH set, then verify that the build completes without requiring the toolchain path to be added separately to PATH.

Written by the indexing model from the issue text.