[to-be-committed] [RISC-V] Use Zbkb for general 64 bit constants when profitable

Basically this adds the ability to generate two independent constants during
synthesis, then bring them together with a pack instruction. Thus we never need
to go out to the constant pool when zbkb is enabled. The worst sequence we ever
generate is

lui+addi+lui+addi+pack

Obviously if either half can be synthesized with just a lui or just an addi,
then we'll DTRT automagically.   So for example:

unsigned long foo_0xf857f2def857f2de(void) {
    return 0x1425000028000000;
}

The high and low halves are just a lui.  So the final synthesis is:

>         li      a5,671088640            # 15    [c=4 l=4]  *movdi_64bit/1
>         li      a0,337969152            # 16    [c=4 l=4]  *movdi_64bit/1
>         pack    a0,a5,a0        # 17    [c=12 l=4]  riscv_xpack_di_si_2

On the implementation side, I think the bits I've put in here likely can be
used to handle the repeating constant case for !zbkb.  I think it likely could
be used to help capture cases where the upper half can be derived from the
lower half (say by turning a bit on or off, shifting or something similar).
The key in both of these cases is we need a temporary register holding an
intermediate value.

Ventana's internal tester enables zbkb, but I don't think any of the other
testers currently exercise zbkb.  We'll probably want to change that at some
point, but I don't think it's super-critical yet.

While I can envision a few more cases where we could improve constant
synthesis,   No immediate plans to work in this space, but if someone is
interested, some thoughts are recorded here:

> https://wiki.riseproject.dev/display/HOME/CT_00_031+--+Additional+Constant+Synthesis+Improvements

gcc/
	* config/riscv/riscv.cc (riscv_integer_op): Add new field.
	(riscv_build_integer_1): Initialize the new field.
	(riscv_built_integer): Recognize more cases where Zbkb's
	pack instruction is profitable.
	(riscv_move_integer): Loop over all the codes.  If requested,
	save the current constant into a temporary.  Generate pack
	for more cases using the saved constant.

gcc/testsuite

	* gcc.target/riscv/synthesis-10.c: New test.

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