arch: Correct the direction of the arch->gem5 lib tag implication.

The arch implies gem5 lib, and not the other way around. Or in other
words, if, for example, x86 is the ISA, the having the tag 'x86 isa'
would imply that that file also has the tag 'gem5 lib'. Having the tag
'gem5 lib' would not imply 'x86 isa'.

This worked out because when testing for a single tag, we were using
with_any_tags, and 'gem5 lib' would expand to 'gem5 lib' and 'x86 isa'.
Then we would match files which were non-specific and used 'gem5 lib',
or files which had more specifically used 'x86 isa' only. Files which
used, for instance, 'arm isa', would not meet either criteria of the
implied "or".

Change-Id: I301d1bbbbcac1594371584d4b0d5d291b7b77fc4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51827
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Daniel Carvalho <odanrc@yahoo.com.br>
1 file changed