mem: fix page_table bug for .fast build

mem: fix page_table bug for .fast build

Since b8b13206c8, the '.fast' build has failed to compile with an error
caused by a variable and an assert.

As a reminder, assert macros are optimized out of the build for '.fast'.
If an assert check requires a variable that is unused anywhere else in
the code, the compiler complains that the variable is unused and the
scons build fails. The solution is to add a M5_VAR_USED specifier to
tell the compiler to ignore the variable.

Change-Id: I38f6bbed1e4c0506c5bbc1206c21f1f7e3d8dfe6
Reviewed-on: https://gem5-review.googlesource.com/8462
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>