[SystemZ][z/OS] Fix f32 variadic argument assertion

The tablegen lines that specify the XPLINK64 calling convention for promoting an f32 vararg to an f64 are effectively overwritten by the following tablegen line which bitcast an f64 vararg to an i64 (so that it can be used in the GPRs). It becomes a bitcast from f32 to i64.

Since we don't handle a bitcast for f32s this caused an assertion.