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.

MFC after:	1 week

lib/libthr/arch/amd64/include/pthread_md.h

@@ -71,7 +71,7 @@ struct tcb {
 	u_long __i;						\
 	__asm __volatile("movq %%fs:%1, %0"			\
 	    : "=r" (__i)					\
-	    : "m" (*(u_long *)(__tcb_offset(name))));		\
+	    : "m" (*(volatile u_long *)(__tcb_offset(name))));  \
 	__result = (__tcb_type(name))__i;			\
 								\
 	__result;						\

lib/libthr/arch/i386/include/pthread_md.h

@@ -70,7 +70,7 @@ struct tcb {
 	u_int __i;						\
 	__asm __volatile("movl %%gs:%1, %0"			\
 	    : "=r" (__i)					\
-	    : "m" (*(u_int *)(__tcb_offset(name))));		\
+	    : "m" (*(volatile u_int *)(__tcb_offset(name))));	\
 	__result = (__tcb_type(name))__i;			\
 								\
 	__result;						\