Make abort messages available to debuggerd.

This adds __libc_fatal, cleans up the internal logging code a bit more,
and switches suitable callers over to __libc_fatal. In addition to logging,
__libc_fatal stashes the message somewhere that the debuggerd signal handler
can find it before calling abort.

In the debuggerd signal handler, we pass this address to debuggerd so that
it can come back with ptrace to read the message and present it to the user.

Bug: 8531731
Change-Id: I416ec1da38a8a1b0d0a582ccd7c8aaa681ed4a29

linker/linker.h

@@ -186,6 +186,7 @@ Elf32_Sym* dladdr_find_symbol(soinfo* si, const void* addr);
 Elf32_Sym* dlsym_handle_lookup(soinfo* si, const char* name);
 
 void debuggerd_init();
+extern "C" abort_msg_t* gAbortMessage;
 extern "C" void notify_gdb_of_libraries();
 
 char* linker_get_error_buffer();