[NVPTX] Use by default 'sm_60' architecture when expanding %ptxas-verify macro.

Also get rid of explicitly specified '-march' values for old architectures.
This simplifies %ptxas-verify statements.
After the change, we can potentially miss cases where a new functionality
is added to the architecture without appropriate checks in the
backend. On the other hand, this is mostly true for old architectures
that have been thoroughly tested.

Differential Revision: https://reviews.llvm.org/D141736