[DARWIN]: Add -mdynamic-no-pic to Darwin-specific options

[Robert Bowdidge <bowdidge at apple dot com>]

By default, the gcc on Darwin generates position-independent code for 
accesses to static and global data on the assumption that an object 
file will be linked into a dynamic library where their absolute 
addresses will not be known til load time.  The code generated is less 
efficient because of the need for calculating the current pc in the 
function prologue.  This option is unneeded for object files linked 
into executables because executables always load at an address known at 
static link time.

This change adds the flag "-mdynamic-no-pic" when compiling gcc itself, 
improving the efficiency of gcc's code and thus improving compile 
times.  On our compiler timing test projects, this change improved 
compile times by 3-5%.  gcc still bootstraps after this change.

Specifically, the change adds the flag to the XCFLAGS variable; 
gcc/Makefile.in states this variable holds options needed for "most 
compilations but not when using the gcc just built".  It correctly adds 
the option to the compiler when bootstrapping, but does not use the 
flag for any of the libraries (libstdc++, libgcc, etc.)

Ok to submit?

Index: gcc/config/rs6000/x-darwin
===================================================================
RCS file: /cvs/gcc/gcc/gcc/config/rs6000/x-darwin,v
retrieving revision 1.5
diff -u -b -d -r1.5 x-darwin
--- gcc/config/rs6000/x-darwin  30 Apr 2003 01:52:06 -0000      1.5
+++ gcc/config/rs6000/x-darwin  16 Aug 2004 23:13:07 -0000
@@ -1,3 +1,4 @@
+XCFLAGS = -mdynamic-no-pic
 host-darwin.o : $(srcdir)/config/rs6000/host-darwin.c $(CONFIG_H) 
$(SYSTEM_H) \
   coretypes.h hosthooks.h hosthooks-def.h toplev.h diagnostic.h
        $(CC) -c $(ALL_CFLAGS) $(ALL_CPPFLAGS) $(INCLUDES) \