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/platform/bionic/mte_kernel.h b/libc/platform/bionic/mte_kernel.h
index 984d17d..d22d65e 100644
--- a/libc/platform/bionic/mte_kernel.h
+++ b/libc/platform/bionic/mte_kernel.h
@@ -51,4 +51,6 @@
 #define SEGV_MTEAERR 8
 #define SEGV_MTESERR 9
 
+#define PTRACE_PEEKTAG 33
+
 #endif