This fixes compilation of the `re` natmod example when built with Picolibc
in the CI environment. Ubuntu 22.04's combination of its bare metal RISC-V
toolchain and its version of Picolibc makes the `alloca` symbol more
elusive than it should be.
This commit makes the `re` natmod try harder to get an `alloca`
implementation.
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>