package/botan: add support for riscv32

package/botan: add support for riscv32

Backport an upstream patch to add support for riscv32. Although this is
a new feature (new arch support), this is an upstream commit, so we can
expect it to be available in a future release.

Fixes:
 - http://autobuild.buildroot.org/results/1c399312dbec5d7a28ec90d62fdd8f47fa14ff4b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - technically, this is not a bug fix, but new arch support
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>