Major audit of all Mesa's x86 assembly code. This round is basically

Major audit of all Mesa's x86 assembly code.  This round is basically
general cleanups - more to come.

Added P6 architecture timing to debug_xform routines.  Still need to add
test_all_vertex_functions test for the v16 asm.  Dynamic reconfiguration
of counter overhead for more accurate benchmarking.

Removed redundant x86 assembly build options, replace with linux-x86 and
so on.  This will build all x86, MMX, 3DNow!, SSE etc assembly into
libGL - preferred over many different options to include some but not
all assembly code.  Note that linux-3dnow and linux-katmai options have
thus been removed.