disable use of arm memcpy asm if building as thumb code

the thumb incompatibilities in the asm are probably only minor and
should be fixable, but for now just use the C version.

adapt build of arm memcpy asm not to use .sub files

this depends on commit 9f5eb77992b42d484d69e879d24ef86466f20f21, which
made it possible to use a .c file for arch-specific replacements, and on
commit 2f853dd6b9a95d5b13ee8f9df762125e0588df5d, 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.