gpiosim: use twalk() instead of twalk_r()

gpiosim: use twalk() instead of twalk_r()

twalk_r() is a GNU extension for the binary search tree API. Musl libc
doesn't provide it and after inquiring with the maintainer it turned out
it won't provide it anytime soon as musl strives to only implement well
standardized functions.

In order to not limit building libgpiosim and tests to glibc only, let's
replace twalk_r() with twalk() and use global variables for the state.
It's not a big deal as we're already using a global root node and a mutex
to protect it.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>