s390x regtest: Adjust vec2_float for Clang

s390x regtest: Adjust vec2_float for Clang

The vec2_float test case doesn't compile with Clang, due to two issues:

* Clang doesn't correctly support `long double' vectors, as documented in
  this GitHub issue: https://github.com/llvm/llvm-project/issues/61291

* llvm-as behaves differently from gas for the reserved field in the
  `.insn vrr' directive.  Instead of inserting the given data from the
  first operand, it initializes the reserved field with zeros.

Circumvent these issues.