in arm rtabi these __aeabi_* functions have special abi (they are
only allowed to clobber r0,r1,r2,r3,ip,lr,cpsr), so they cannot
be simple wrappers around normal string functions (which may
clobber other registers), the safest solution is to write them in
asm, a minimalistic implementation works because these are not
supposed to be emitted by compilers or used in general.
this is possible with the new build system that allows src/*/$(ARCH)/*
files which do not shadow a file in the parent directory, and yields a
more logical organization. eventually it will be possible to remove
arch/*/src from the build system.
this depends on commit 9f5eb77992, which
made it possible to use a .c file for arch-specific replacements, and on
commit 2f853dd6b9, the out-of-tree build
support, which made it so that src/*/$(ARCH)/* 'replacement' files get
used even if they don't match the base name of a .c file in the parent
directory.
