#455 __sdcc_external_startup return checking for PDK platform

[Basil Hussain]

A patch to implement checking of return value of __sdcc_external_startup and skip variables initialisation if non-zero on the PDK platforms (pdk13, pdk14, pdk15).

Regression tests all pass:

Summary for 'pdk14': 0 failures, 5780 tests, 3281 test cases, 1392188 bytes, 14176600 ticks
Summary for 'pdk15': 0 failures, 11051 tests, 3292 test cases, 2760589 bytes, 37640240 ticks

I was unable to run the regression tests for pdk13, as all it seems to do when building the test cases is erupt into a tidal wave of errors, so I decided to skip it. I have independently verified correct operation of the changes there using the same test scripts/program as per patch [#453].

[Philipp Klaus Krause]

Applied in [r13802].