RISC-V: Fix missing boolean_expression in zmmul extension

RISC-V: Fix missing boolean_expression in zmmul extension

Update v1->v2
    Add testcase for this patch.

Missing boolean_expression TARGET_ZMMUL in riscv_rtx_costs() cause different instructions when
multiplying an integer with a constant. ( https://github.com/riscv-collab/riscv-gnu-toolchain/issues/1482 )

int foo(int *ib) {
    *ib = *ib * 33938;
    return 0;
}

rv64im:
        lw      a4,0(a1)
        li      a5,32768
        addiw   a5,a5,1170
        mulw    a5,a5,a4
        sw      a5,0(a1)
        ret

rv64i_zmmul:
        lw      a4,0(a1)
        slliw   a5,a4,5
        addw    a5,a5,a4
        slliw   a5,a5,3
        addw    a5,a5,a4
        slliw   a5,a5,3
        addw    a5,a5,a4
        slliw   a5,a5,3
        addw    a5,a5,a4
        slliw   a5,a5,1
        sw      a5,0(a1)
        ret

Fixed.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_rtx_costs): Add TARGET_ZMMUL.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zmmul-3.c: New test.