Fix a strange macro re-definition compile error. If the VM_MAXUSER_ADDRESS

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).

Nothing has changed about this code for a while but the compile error is
new, so this must be fallout from the clang 3.7 update or something.