scons,arch: Make the gem5 lib tag imply the current arch tag.

That way you can use the tag to specify when a source file should be
built, instead of conditionally declaring the file to SCons.

Change-Id: Ia3a23860d2ee39ec6b32ee2195648b4d88564c83
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50330
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/SConscript

@@ -55,6 +55,8 @@ Import('*')
 #
 #################################################################
 
+env.TagImplies('gem5 lib', env.subst('${TARGET_ISA} isa'))
+
 env.SwitchingHeaders(
     Split('''
         decoder.hh