wayland-egl: use correct `nm` path when cross-compiling

Inspired by Heiko Becker and Eric's work in libdrm and Mesa
respectively.

Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>