Add a macro to annotate function end and start using both ENTRY and END
for each function. This allows valgrind (and presumably other debugging
tools) to use the debug symbols to trace the functions.
Change-Id: I5f09cef8e22fb356eb6f5cee952b031e567599b6
Only provide an implementation for ARM at the moment, since
it requires specific assembly fragments (the standard syscall
stubs cannot be used because the child returns in a different
stack).
