build runtime for embedded Linux (cross)

[Dmitry Ponyatov]

What's can be wrong with `i686-linux-musl` cross target for 
`ldc-build-runtime` ?

```sh
ponyatov at debian:~/player$ rm -rf tmp/ldc_i686-linux-musl ; 
CORES=1 make ldc
```
```
PATH=/home/ponyatov/player/host/bin:/opt/ldc2-1.32.0-linux-x86_64/bin:/home/ponyatov/.cargo/bin:/home/ponyatov/.npm/bin:/home/ponyatov/.opam/default/bin:/home/ponyatov/.nimble/bin:/home/ponyatov/Arduino/arduino-ide_2.2.1_Linux_64bit/resources/app/lib/backend/resources:/home/ponyatov/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/bin:/home/ponyatov/Arduino/arduino-ide_2.2.1_Linux_64bit:/usr/lib/jvm/java-11-openjdk-amd64/bin:/home/ponyatov/.cache/rebar3/bin:/home/ponyatov/bin:/home/ponyatov/.local/bin:/usr/local/bin:/usr/bin:/bin:/usr/games CC=i686-linux-musl-gcc CXX=i686-linux-musl-g++ /opt/ldc2-1.32.0-linux-x86_64/bin/ldc-build-runtime -j1 --ldc /opt/ldc2-1.32.0-linux-x86_64/bin/ldc2  \
         --buildDir /home/ponyatov/player/tmp/ldc_i686-linux-musl 
--ldcSrcDir /home/ponyatov/player/tmp/ldc-1.32.0-src \
         --targetSystem i686-linux-musl BUILD_SHARED_LIBS=ON \
         --dFlags="-mtriple=i686-linux-musl;-mcpu=i686"
```
```
.: Creating build directory: 
/home/ponyatov/player/tmp/ldc_i686-linux-musl
.: Invoking: cmake 
-DLDC_EXE_FULL=/opt/ldc2-1.32.0-linux-x86_64/bin/ldc2 
-DDMDFE_MINOR_VERSION=102 -DDMDFE_PATCH_VERSION=2 
-DTARGET_SYSTEM=i686-linux-musl 
'-DD_EXTRA_FLAGS=-mtriple=i686-linux-musl;-mcpu=i686' 
-DBUILD_SHARED_LIBS=ON 
/home/ponyatov/player/tmp/ldc-1.32.0-src/runtime
-- The C compiler identification is GNU 12.3.0
-- Check for working C compiler: 
/home/ponyatov/player/host/bin/i686-linux-musl-gcc
-- Check for working C compiler: 
/home/ponyatov/player/host/bin/i686-linux-musl-gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
```
```
CMake Error at CMakeLists.txt:111 (message):
   Shared libraries (BUILD_SHARED_LIBS) are only supported on 
Windows, Linux,
   macOS, FreeBSD and DragonFly for the time being.
```