Makefile: make it possible to build arch specific C files
arm: add __aeabi_atexit()

arm eabi requires this symbol for static C++ dtors.
usually it is provided by libstdc++, but when a C++ program
doesn't use the std lib (free-standing), the libc has to provide
it.
this was encountered while building transmission, which
depends on such a C++ library (libutp).

this function is nearly identical to __cxa_atexit, but it has the
order of argumens swapped for "performance reasons".
see page 25 of

 http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf

there are other aeabi specific C++ support functions missing, but
it is not clear yet that GCC makes use of them so we omit them for
the moment.
improve error strings

this definitely has the potential to be a bikeshed topic, so some
justification is in order. most of the changes made fit into one of
the following categories:
1. alignment with text in posix, xsh 2.3
2. eliminating overly-specific text for shared error codes
3. making the message match more closely with the macro name
4. removing extraneous words

in particular, the EAGAIN/EWOULDBLOCK text is updated to match the
description of EAGAIN (which covers both uses) rather than saying the
operation would block, and ENOTSUP/EOPNOTSUPP is updated not to
mention sockets.

the distinction between ENFILE/EMFILE has also been clarified; ENFILE
is aligned with the posix text, and EMFILE, which lacks concise posix
text matching any historic message, is updated to emphasize that the
exhausted resource is not open files/open file descriptions, but
rather the integer 'address space' of file descriptors.

some messages may be further tweaked based on feedback.
fix typo in newly-added error message for EOWNERDEAD
Merge branch 'master' of git://git.musl-libc.org/musl into g_hack