Move targetHasSVE function to lldbtest.py

targetHasSVE helper function was added to test for availability of SVE support
by connected platform. We now intend to use this function in other testcases
and I am moving it to a generic location in lldbtest.py to allow usage by
other upcoming testcases.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D86872