sys-libs/libunwind: unkeyword riscv

sys-libs/libunwind: unkeyword riscv

There doesn't seem to be any official support for RISC-V yet. The
build fails with:

  checking for ELF helper width... configure: error: Unknown ELF target: riscv64

The git HEAD (commit a36ec8cfdb8764e4f8bf6b16a149a60ea6ad038d) doesn't
seem to provide support for RISC-V either:

AC_MSG_CHECKING([for ELF helper width])
case "${target_arch}" in
(arm|hppa|ppc32|x86|sh) use_elf32=yes; AC_MSG_RESULT([32]);;
(aarch64|ia64|ppc64|x86_64|s390x|tilegx)  use_elf64=yes; AC_MSG_RESULT([64]);;
(mips)                 use_elfxx=yes; AC_MSG_RESULT([xx]);;
*)                     AC_MSG_ERROR([Unknown ELF target: ${target_arch}])
esac

This reverts commit 9037f242c1d5941a57612f6b617cc74b761e6896.

Signed-off-by: Göktürk Yüksek <gokturk@gentoo.org>