[MSan] Fix calling pointers to varargs functions on SystemZ

VarArgSystemZHelper.visitCallBase() checks whether the callee has the
"use-soft-float" attribute, but if the callee is a function pointer, a
null pointer dereference happens.

Fix by checking this attribute on the current function. Alternatively,
one could try the callee first, but this is pointless, since one should
not be mixing hardfloat and softfloat code anyway.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D143296