I report the successfull build of the cross-compiler 
x86_64-musl-gcc-6.3.0 - understanding Ada - on x86_64 Debian Wheezy.

     The toolchain for build was gcc-6.3.0 for Linux-Gnu provided by 
Adacore.The build toolchain to build major version 6 must be at least 
major version 5.

     Of course, the language option must be '--enable-languages=ada,c,c++' .

     Three patches were necessary:
        gnat63-terminals.diff: removes a #include <termio.h> in a C 
source file.
        gnat63-prj-attr.diff: corrects the layout of a record in an Ada 
source file to prevent a warning concerning performance.
        gnat63-xr_tabls.diff: same as the previous.

     Note on static linking:
     Compilation of Ada program is often made in one invocation of 
gnatmake, which calls gcc, gnatbind and gnatlink. By default, gnatmake 
links everything statically, except the libc (probably a gnuism). If the 
option -shared is given, then it links everything dynamically. The 
option -static is ignored by gnatmake.
     However, it is possible to produce static executables by invoking 
gcc -c, gnatbind, andgnatlink, because gnatlink honours the -static option.

     I attach the patches in case someone is interested.

     I was able to compile a native x86_64-pc-linux-musl-gcc-6.3.0 (with 
the 3 steps) using this cross-compiler for build.

                 Didier