Reorganise CPU handling for Sparc. When using -mcpu=v9 and co, __sparcv8

is not defined for 32bit mode, but __sparcv9 is. Pass down the correct
-target-cpu flags to the backend, so that instruction restrictions are
applied correctly. Pass down the correct -A flag when not using IAS.
The latter is limited to NetBSD targets in this commit.

llvm-svn: 252545