Expose libc_shared_globals to libc.so with symbol

Previously, the address of the global variable was communicated from the
dynamic linker to libc.so using a field of KernelArgumentBlock, which is
communicated using the TLS_SLOT_BIONIC_PREINIT slot.

As long as this function isn't called during relocations (i.e. while
executing an ifunc), it always return a non-NULL value. If it's called
before its PLT entry is relocated, I expect a crash.

I removed the __libc_init_shared_globals function. It's currently empty,
and I don't think there's one point in libc's initialization where
shared globals should be initialized.

Bug: http://b/25751302
Test: bionic unit tests
Change-Id: I614d25e7ef5e0d2ccc40d5c821dee10f1ec61c2e
diff --git a/linker/ld_android.cpp b/linker/ld_android.cpp
index 0528cd8..152c2e2 100644
--- a/linker/ld_android.cpp
+++ b/linker/ld_android.cpp
@@ -53,6 +53,7 @@
 __strong_alias(__loader_dlvsym, __internal_linker_error);
 __strong_alias(__loader_add_thread_local_dtor, __internal_linker_error);
 __strong_alias(__loader_remove_thread_local_dtor, __internal_linker_error);
+__strong_alias(__loader_shared_globals, __internal_linker_error);
 #if defined(__arm__)
 __strong_alias(__loader_dl_unwind_find_exidx, __internal_linker_error);
 #endif