Fix a strange macro re-definition compile error.  If the VM_MAXUSER_ADDRESS
value is defined as a config option the definition is emitted into
opt_global.h which is force-included into everything.  In addition, the
symbol is emitted by the genassym mechanism, but that by its nature reduces
the value to a 0xnnnnnnnn number.  When compiling a .S file you end up
with two different definitions of the macro (they evaluate to the same
number, but the text is different, upsetting the compiler).