xlc: switch to clang-style warning disabling

Recently the jdk/jdk build on AIX switched to xlc16/xlclang.

This means we can now use the standard clang-style warning disabling (the old legacy qsuppress warning disablings from legacy xlc
might not fully work any more).

I think we are pretty close in reducing the compiler warnings to 0 on AIX,
which would make it possible to build with warnings as errors at some point in the future .

In a first step I would disable these warnings in HS for xlc16/xlclang :

tautological-compare : (seen in os_aix.cpp)
shift-negative-value : (seen in c1_LIRGenerator_ppc.cpp)