Fix PAL test build on non-Linux Unix platforms

[am11]

A step towards re-enabling PAL tests in the CI: #34776.

On macOS and Illumos/SmartOS, currently the C11's non-optional header <uchar.h> (not to be confused with ICU's unicode/uchar.h) is not available.

This PR removes the uchar.h usage from PAL tests (which is used for the definition of char16_t) and conditionally declare char16_t for "C sources in PAL tests".

All tests are passing on macOS baremetal, one test failed on Ubuntu x64 virtual machine:

<test name="miscellaneous.queryperformancecounter.test1" type="miscellaneous.queryperformancecounter" method="test1" result="Fail" >
  <failure exception-type="Exit code: 1">
    <message><![CDATA[ERROR:  average diff 59 acceptable 15.]]></message>
  </failure>
</test>

There are currently six failures on SmartOS x64 VM (http://sprunge.us/3eVz48 - file: /tmp/PalTestOutput/default/pal_tests.xml) with #35173 (and libunwind) changes.

cc @janvorli, @rmustacc