Itanium HP-UX build failure, register stack

The code we added recently to the stack-depth check to also check the 
register stack on ia64 doesn't compile on HP-UX B.11.31, using the HP 
aCC compiler:

cc -Ae +O2 -g -I../../../src/include -D_XOPEN_SOURCE_EXTENDED   -c -o 
postgres.o postgres.c
"postgres.c", line 3002: warning #2837-D: omission of explicit type is          nonstandard ("int" assumed)  static
__inline__char *         ^
 

"postgres.c", line 3002: error #2065: expected a ";"  static __inline__ char *                    ^

I committed a fix for that using the HP-UX specific _Asm_mov_from_ar 
macro, similar to how we use _Asm_xchg for TAS().

With this, 8.4 compiles on HP-UX again, yeaee! 9.0 still has this issue: 
http://archives.postgresql.org/pgsql-hackers/2010-11/msg01971.php. I'll 
take a look at that..

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com