ANDROID: GKI: Add special linux_banner_ptr for modules

We can't modify the original linux_banner symbol due to ramdump parsing
dependencies. So add an additional symbol pointer that just points at
it. This way ABI monitoring won't care about the value and
modules can still access the banner value.

Signed-off-by: Will McVicker <willmcvicker@google.com>
Bug: 154586724
Bug: 154345163
Test: compile
Change-Id: I65678e02467b8754824a7527270a725741394fe3