RISC-V: Fix .cfi_offset directive when push/pop in zcmp

The incorrect cfi directive info breaks stack unwind in try/catch/cxa.

Before patch:
  cm.push	{ra, s0-s2}, -16
  .cfi_offset 1, -12
  .cfi_offset 8, -8
  .cfi_offset 18, -4

After patch:
  cm.push	{ra, s0-s2}, -16
  .cfi_offset 1, -16
  .cfi_offset 8, -12
  .cfi_offset 9, -8
  .cfi_offset 18, -4

gcc/ChangeLog:

	* config/riscv/riscv.cc: Set multi push regs bits.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/zcmp_push_gpr.c: New test.