[RISCV] Add RV64 CHECK lines to test/CodeGen/RISCV/vararg.ll and prepare for hard float tests

vararg.ll previously missed RV64 tests. This patch also prepares for using
vararg.ll to test handling of varargs for the ilp32f/ilp32d/lp64f/lp64d hard
float ABIs. In these ABIs, varargs are passed as in either the ilp32 or lp64
ABI. Due to some slight codegen differences, different check lines are needed
for when RV32D is enabled.

llvm-svn: 357350