Fix #4177: RyuJIT/x86 NYI related to lvaArgType()

Fix #4177: RyuJIT/x86 NYI related to lvaArgType()

For x86, this function can simply return the variable type. There is
no need to map struct types. The function actually will never be
called on x86 for struct types because it is only called for register
arguments.

This NYI was only hit in the case of JMP calls. I fixed two cases
in the JMP implementation where FEATURE_VARARGS code assumed AMD64, due
to the Windows AMD64 ABI convention to pass float varargs values in both
integer and floating point registers. I put that under `#ifdef _TARGET_AMD64_`.

This removes 426 NYI.