Hexagon: add target builtins to kernel

Add the compiler-rt builtins like memcpy to the hexagon kernel.

Signed-off-by: Sid Manning <sidneym@codeaurora.org>
Add SYM_FUNC_START/END, ksyms exports
Signed-off-by: Brian Cain <bcain@codeaurora.org>

Tested-by: Nick Desaulniers <ndesaulniers@google.com>