linker: enable -fvisibility=hidden

Compile the linker with -fvisibility=hidden. This reduces the number
of symbols that show up in the .dynsym section of the linker.
These symbols are never exported to other applications.

In particular, this fixes a problem with setting -DLINKER_DEBUG=1
introduced in 468319ce4f3f7383d788b76c09cda2a405311f36.
Because the symbols "debug_verbosity" and "format_log" have not been
resolved before the linker links itself, any attempt to call
PRINT / INFO / TRACE / WARN / ERROR will result in a segfault.
This change allows the static linker to produce a relative reference
to these symbols rather than relying on relocation.

This also has a nice side effect of making the linker slightly smaller
and slightly more optimized.

The following symbols no longer in the .dynsym section of the linker
after this change:

-addr_to_name
-call_constructors_recursi
-calloc
-debugger_init
-debugger_signal_handler
-debug_verbosity
-dladdr
-dlclose
-dlerror
-dl_lock
-dlopen
-dlsym
-dl_unwind_find_exidx
-find_containing_library
-find_containing_symbol
-find_library
-format_buffer
-free
-libdl_info
-linker_env_get
-linker_env_init
-linker_env_secure
-linker_env_unset
-linker_get_error
-__linker_init
-lookup
-lookup_in_library
-malloc
-notify_gdb_of_libraries
-notify_gdb_of_load
-notify_gdb_of_unload
-realloc
-rtld_db_dlactivity
-unload_library
-vsnprintf

Bug: 5827809
Change-Id: I5e8cd7dcf48c1d6831a970a67f63f24916c5e437

linker/Android.mk

@@ -12,7 +12,9 @@ LOCAL_SRC_FILES:= \
 
 LOCAL_LDFLAGS := -shared
 
-LOCAL_CFLAGS += -fno-stack-protector -Wstrict-overflow=5
+LOCAL_CFLAGS += -fno-stack-protector \
+        -Wstrict-overflow=5 \
+        -fvisibility=hidden
 
 # Set LINKER_DEBUG to either 1 or 0
 #