Wrong extension when cross-compiling a shared library

I have a multi-platform library declared in buck that can be compiled for iOS, Android and OSX. The build process is hosted on an OSX machine and compiles fine for the mentioned platforms. I built an Ubuntu 16.04 x86_64 toolchain using crosstool-ng. I was able to use it with the following .buckconfig configuration.

[cxx#crossubuntu]
 cpp = /Volumes/Untitled/x-tool/x86_64-ubuntu16.04-linux-gnu/bin/x86_64-ubuntu16.04-linux-gnu-gcc
 cc = /Volumes/Untitled/x-tool/x86_64-ubuntu16.04-linux-gnu/bin/x86_64-ubuntu16.04-linux-gnu-gcc
 cxxpp = /Volumes/Untitled/x-tool/x86_64-ubuntu16.04-linux-gnu/bin/x86_64-ubuntu16.04-linux-gnu-g++
 cxx = /Volumes/Untitled/x-tool/x86_64-ubuntu16.04-linux-gnu/bin/x86_64-ubuntu16.04-linux-gnu-g++
 ld = /Volumes/Untitled/x-tool/x86_64-ubuntu16.04-linux-gnu/bin/x86_64-ubuntu16.04-linux-gnu-g++
 linker_platform = GNU
 default_platform = linux-x86_64

Then calling buck build //src/mylib:mylib#crossubuntu,shared

The resulting library is a valid ELF file but the filename is terminated with ".dylib". I can simply add a genrule to rename it but it would be nice to have a .so generated instead since I declared the default_platform to be linux-x86_64.

Another strange thing is that if instead of the crossubuntu flavor I use linux-x86_64, it does not work.

[cxx#linux-x86_64]
 cpp = ...
 ...
 default_platform = linux-x86_64

buck build //src/mylib:mylib#linux-x86_64,shared fails because it is not able to find the linux-x86_64 platform...