Fix a data race in pocl_runtime_config.c
Fix a data race in pocl_runtime_config.c

LL_PREPEND(head, item) changes the head (env_cache) on every call,
which conflicts with other threads calling find_env() with env_cache.

pocl/pocl,franz/pocl,franz/pocl,pocl/pocl,Oblomov/pocl,pocl/pocl,Oblomov/pocl,Oblomov/pocl,pocl/pocl,pocl/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl,franz/pocl,franz/pocl