fix error when cross-compiling a static MinGW build with --icu

ICU uses different library names for Windows depending on whether
linkage is static or shared, which is reflected in config.tests/unix/icu
and src/corelib/tools/tools.pri. However, it is not possible to determine
the type of linkage in the config test as that information is set later
in the configure script.

To fix this, "-config static" is additionally passed to compileTest
when cross-compiling for a static MinGW target.

Change-Id: I22c2d543ea14feebbad36961c9a29e99f66323ff