riscv: selftests: Add signal handling vector tests

Add two tests to check vector save/restore when a signal is received
during a vector routine. One test ensures that a value is not clobbered
during signal handling. The other verifies that vector registers
modified in the signal handler are properly reflected when the signal
handling is complete.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Andy Chiu <andy.chiu@sifive.com>
Tested-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20240403-vector_sigreturn_tests-v1-1-2e68b7a3b8d7@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>