We're using the ARM Embedded ABI (AEABI) to build our code for the Cortex-M.
  That ABI specifies a lot of things: how procedures are called, how ELF files
  should be structured, and last but not least, a run-time.
  
  That run-time consists in a list of helper functions that the compiler is free
  to call anywhere : unaligned memory accesses, integer division functions,
  memory copying/clearing/setting, etc...
  
  Since we're telling our compiler to build an AEABI binary, it may decide to use
  those symbols, and so we have to provide an implementation for them. Refer to
  the "Run-time ABI for the ARM Architecture" for a full list of functions.
  http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf
  
  Note that this is not formally the job of a libc implementation. Similar code is
  often shipped alongside a compiler (LLVM calls it compiler-rt, GCC libgcc). But
  since we're using so few symbols in practice, it makes sense to glue it in. Also
  some libc do also implement this (newlib, for example).