[SystemZ] Improve handling of inline asm constraints.

The "{=v0}" constraint did not result in the expected error message in the
abscence of the vector facility, because 'v0' matches as a string into the
AnyRegBitRegClass in common code.

This patch adds checks for vector support in case of "{v" and soft-float in
case of "{f" to remedy this.

Review: Ulrich Weigand.