[NVPTX] Unforce minimum alignment of 4 for byval arguments of device-side functions.

Minimum alignment of 4 for byval arguments was forced to workaround
a bug in old versions of ptxas. Details: https://reviews.llvm.org/D22428.
Recent ptxas versions (> 9.0) do not seem to have this bug, so alignment
requirement was relaxed. To force again minimum alignment of 4, use
'-force-min-byval-param-align' option.