MFC r228536:

MFC r228536:

The TCB_GET32() and TCB_GET64() macros in the i386 and amd64-specific
versions of pthread_md.h have a special case of dereferencing a null
pointer.  Clang warns about this with:

In file included from lib/libthr/arch/i386/i386/pthread_md.c:36:
lib/libthr/arch/i386/include/pthread_md.h:96:10: error: indirection of non-volatile null pointer will be deleted, not trap [-Werror,-Wnull-dereference]
	return (TCB_GET32(tcb_self));
		^~~~~~~~~~~~~~~~~~~
lib/libthr/arch/i386/include/pthread_md.h:73:13: note: expanded from:
	    : "m" (*(u_int *)(__tcb_offset(name))));            \
		   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/libthr/arch/i386/include/pthread_md.h:96:10: note: consider using __builtin_trap() or qualifying pointer with 'volatile'

Since this indirection is done relative to the fs or gs segment, to
retrieve thread-specific data, it is an exception to the rule.

Therefore, add a volatile qualifier to tell the compiler we really want
to dereference a zero address.

git-svn-id: svn://svn.freebsd.org/base/stable/9@229047 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f