lavu/riscv: do not fallback to AT_HWCAP auxillary vector

If __riscv_hwprobe() fails, then the kernel version is presumably too
old. There is not much point falling back to the auxillary vector.

- The Linux kernel requires I, so the flag is always set on Linux, and
  run-time detection is unnecessary. Our RISC-V assembler does anyway not
  support targets without I.

- Linux can compile with or without F and D, but it cannot perform
  run-time detection for them (a kernel with F support will not boot a
  processor without F). The run-time detection is thus useless in that
  case. Besides F and D extensions are used throughout the C code, so
  their run-time detection would not be practical.

- Support for V was added in a later kernel version than riscv_hwprobe(),
  so the system call will always be available if the kernel supports V.
  The only exception would be vendor kernel forks, but those are known to
  haphasardly pretend to support V on systems without actual V support, or
  with only pre-ratification binary-incompatible version. Furthermore, a
  large chunk of our optimisations require Zba and/or Zbb which cannot be
  detected with HWCAP in those kernels.

For what it is worth, OpenJDK already took a similar action. Note that this
keeps AT_HWCAP usage for platforms with neither C run-time <sys/hwprobe.h>
nor kernel <asm/hwprobe.h>, notably kernels other than Linux.

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