Share header containing assembler macros with userspace

asm.h is needed by tests in libc-trusty so this change moves it to
the shared include directory (external/include/shared/lk).

Bug: None
Change-Id: I11601a8bc8d314408f0535f1048348ffe397f8c9
diff --git a/include/asm.h b/include/shared/lk/asm.h
similarity index 100%
rename from include/asm.h
rename to include/shared/lk/asm.h