security/botan2: fix build on powerpc64

This port detects architecture based on uname -m and on powerpc64 it prints powerpc, which makes configure script thinks it builds on 32-bit powerpc and causes error because the compiler is configured for 64-bits.

Explicitly configure as ppc64 on powerpc64.

PR: 241375
Approved by: tremere@cainites.net (maintainer), linimon (mentor)