Fix a build failing in the x86 linux builds ...

Fix a build failing in the x86 linux builds ...

If NaCl is disabled, we get a build failure in the
x86 Linux builds when trying to build monacl_shell
(which isn't defined).

This patch fixes the issue by not trying to include
the gn_migration targets *and* by not trying to
build monacl_shell when nacl is disabled, both of
which were wrong.

Also, this patch fixes an issue in 'analyze' where it
was trying to analyze build_utf8_validator_tables
and not #host.

TBR=brettw@chromium.org
BUG=462945, 461019

Review URL: https://codereview.chromium.org/966223002

Cr-Commit-Position: refs/heads/master@{#318626}