community/ldc: fails to build against GCC11

ldc fails to build on x86_64 with:

  /usr/lib/gcc/x86_64-alpine-linux-musl/11.2.1/../../../../lib/libgphobos.a(fiber.o):
  in function `fiber_switchContext':

  (.text.fiber_switchContext+0x3b): undefined reference to `swapcontext'

and similar for getcontext and makecontext. gdc needs to be fixed to use libucontext instread.

community/ldc has been disabled to unblock the builders