ptrace: decode note segment descriptor types

* configure.ac (AC_CHECK_HEADERS): Add elf.h.
* process.c: Include elf.h.
(nt_descriptor_types): New xlat structure.
(sys_ptrace): Use it.
diff --git a/configure.ac b/configure.ac
index 09056a5..998324d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -201,6 +201,7 @@
 AC_CHECK_HEADERS(m4_normalize([
 	asm/cachectl.h
 	asm/sysmips.h
+	elf.h
 	inttypes.h
 	ioctls.h
 	libaio.h