[builtins][test] Skip scalbn rounding tests on newlib

The picolib/newlib implementation of scalbn gives slightly different
results compared to glibc and compiler-rt's inlined
__compiler_rt_scalbn in certain rounding modes. Since these tests
are already disabled for msvc which doesn't respect the mode change,
this patch skips them for newlib as well.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D150280