[testsuite] [riscv] xfail ssa-dom-cse-2 on riscv64

[testsuite] [riscv] xfail ssa-dom-cse-2 on riscv64

For the same reasons that affect alpha and other targets,
gcc.dg/tree-ssa/ssa-dom-cse-2.c fails to be optimized to the expected
return statement: the array initializer is vectorized into pairs, and
DOM cannot see through that.

Add riscv*-*-* to the list of affected lp64 platforms.  riscv32 is
not affected.

for  gcc/testsuite/ChangeLog

	* gcc.dg/tree-ssa/ssa-dom-cse-2.c: XFAIL on riscv lp64.