Fix recent rvv/base/spill testcase failures

he core issue is we're expecting the frame to have a constant size, but it
doesn't.  So when using the to_constant method we abort.

The safest thing to do is to set no shrink-wrapping components when the
frame size is not fixed.  We might be able to do better later -- iff we
know the offset to the GPRs/FPRs is fixed and fits into the appropriate
number of bits.

Bootstrapped and regression tested (C-only) on riscv64-linux-gnu.  As
expected, it fixes a bucketload of failures in rvv/base/spill-*.c.

gcc/
	* config/riscv/riscv.cc (riscv_get_separate_components): Do not
	do shrink-wrapping for a frame with a variable size.