base: Add M5 flag for [[nodiscard]] attribute

base: Add M5 flag for [[nodiscard]] attribute

This change adds the M5_NODISCARD keyword to allow use of the
[[nodiscard]] attribute with compilers that support C++17. Currently,
C++17 is not a requirement and therefore the M5_NODISCARD has not
effect and does not break compilation for older compilers.

Change-Id: Ifc5c8f34764da3c7291066dcb2ff908c97738c3d
Reviewed-on: https://gem5-review.googlesource.com/10441
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>