package/guile: fix build on riscv32

Fix the following build failure on riscv32:

system/base/target.scm:132:16: In procedure triplet-pointer-size:
unknown CPU word size "riscv32"

Fixes:
 - http://autobuild.buildroot.org/results/6705630c1484239ec8b73d57ebc2e2570fbfc8f8



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>