[lldb] [test] Skip AVX lldb-server test on non-x86 architectures

Skip the AVX-related lldb-server test on non-x86 architectures, as they
do not support AVX.  While technically the test worked on Linux because
the AVX check would simply return false, other platforms do not provide
such a straightforward way of checking for AVX (especially remotely),
and the results of such check may need to be interpreted specially
for the platform in question.

Differential Revision: https://reviews.llvm.org/D97450