arch: Consolidate common debug flags.

Consolidate debug flags which are defined separately in multiple ISAs in
arch/SConscript instead. This will prevent conflicts when more than one
ISA can be compiled at a time.

Change-Id: I70862c403d19f04bbcbabd74dd751ca92d1ed942
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50334
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/arch/riscv/SConscript

@@ -71,7 +71,6 @@ if env['TARGET_ISA'] == 'riscv':
     SimObject('RiscvTLB.py')
 
     DebugFlag('RiscvMisc')
-    DebugFlag('TLBVerbose')
     DebugFlag('PMP')
 
     # Add in files generated by the ISA description.