* configure.host: Set have_crt0 to no for Arm targts when not
providing syscalls. Set sys_dir=arm unconditionally.
Default have_crt0 based on sys_dir.
* configure.in: Use have_crt0.
* libc/configure.in: Ditto.
* libc/sys/configure.in: Ditto.
* configure: Regenerate.
* libc/configure: Regenerate.
* libc/sys/configure: Regenerate.
* libc/sys/arm/Makefile.am (lib_a_SOURCES): Add aeabi_atexit.c.
Only build other files when providing syscalls.
* libc/sys/arm/Makefile.in: Regenerate.
* libc/sys/arm/aeabi_atexit.c: New file.
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