devel/arm-none-eabi-newlib: fix the build with newer clang

Taken from upstream commit 7bfa24c49563717426d1ee237f29d1ccfc4b51db,
cast these two to stinst_type explicitly to fix the build.

(cherry picked from commit d8dd1f531d8f6df9089074d166dbaa02c8aff7d3)