hw/riscv/iommu: fix build error with clang

hw/riscv/iommu: fix build error with clang

Introduced in 0c54acb8243, "hw/riscv: add RISC-V IOMMU base emulation".

../hw/riscv/riscv-iommu.c:187:17: error: redefinition of '_pext_u64'

  187 | static uint64_t _pext_u64(uint64_t val, uint64_t ext)

      |                 ^

D:/a/_temp/msys64/clang64/lib/clang/18/include/bmi2intrin.h:217:1: note: previous definition is here

  217 | _pext_u64(unsigned long long __X, unsigned long long __Y)

      | ^

After a conversation on the mailing list, it was decided to rename and
add a comment for this function.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241104222225.1523751-1-pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>