I started off copying the x86_64 port, getting static build to work
and then iteratively fixing problems detected by libc-testsuite.
currently not containing any of the optimized math and string asm.
at this point all tests except math pass, and both static and dynamic
linking work.

there are a couple of intrusive changes into non-arch specific headers,
caused by mismatch of 32bit longs to "kernel" longs (64bit) used in
some structures and as syscall arguments.

especially the current changes to src/internal/syscall.h are very ugly.

discussion is open for better/cleaner ways to get the same
functionality.

something that didn't make it into this patch is that the arch triple
"x86_64-x32-..." should also be detected by the configure script,
because this is a triple that magically let's binutils and gcc build the
right thing (gcc needs an additional multilib setting to work tho,
--with-multilib-list=mx32). having the arch name itself be x32 seems
cleaner though, especially with regard to the musl-gcc patches in
Gregor's repo that already have ld-musl-x32.so.1 hardcoded as dynlinker.