Add PTRACE_PEEKTAG to mte_kernel.h.

This constant is used by the experimental ptrace API for reading
tags in other processes.

Also expose the platform headers to libunwindstack.

Bug: 135772972
Change-Id: I2daecdaa4c8b557b42222f65fc6b68d13da84ad4
diff --git a/libc/Android.bp b/libc/Android.bp
index 650726a..8d70185 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1751,6 +1751,7 @@
         "//external/perfetto:__subpackages__",
         "//external/scudo:__subpackages__",
         "//system/core/debuggerd:__subpackages__",
+        "//system/core/libunwindstack:__subpackages__",
         "//system/memory/libmemunreachable:__subpackages__",
     ],
     host_supported: true,