arch: Decentralize the arch tag TagImplies in arch/SConscript.

Put the TagImplies for each ISA in its own SConscript.

Change-Id: I90d72aa6f493ac2c27ec54e3ac02ff8151e2f518
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52494
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/riscv/SConscript

@@ -43,6 +43,9 @@
 
 Import('*')
 
+if env['USE_RISCV_ISA']:
+    env.TagImplies('riscv isa', 'gem5 lib')
+
 Source('decoder.cc', tags='riscv isa')
 Source('faults.cc', tags='riscv isa')
 Source('isa.cc', tags='riscv isa')