[libunwind] Remove static_assert / __has_feature macros

The static_assert macro broke on GCC when a scope had two asserts and a
condition that depended on a template parameter. Remove the macro and
rely on the compiler's C++11 static_assert feature.

The __has_feature macro was only used here to determine whether to
define the static_assert macro.

Reviewed By: mstorsjo, #libunwind

Differential Revision: https://reviews.llvm.org/D86411