I've created a patch that adds a patch from 5.5.33 that allows MariaDB to compile with Bison 3.0.4:
https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1045513

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/be7e0f36733e822bf7df90fcfc60f0cf9485e44c

commit be7e0f36733e822bf7df90fcfc60f0cf9485e44c
Author: Casey Bowman <casey.g.bowman@intel.com>
Date: Thu May 24 07:23:15 2018

mariadb: Add Bison 3.0.4 compatibility

Bison 3.0.4 removes some deprecated directives, which breaks mariadb
5.5.32. This introduces a patch from mariadb 5.5.33 that replaces
the deprecated directives with updated directives.

Note that this is still compatible with Bison 2.7.1.

BUG=chromium:731228
TEST='sudo emerge mariadb' works with Bison 3.0.4 and Bison 2.7.1

Change-Id: Ia90d470152d0d5e1e164c203d48ac7eee3be38cf
Signed-off-by: Casey Bowman <casey.g.bowman@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/1045513
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Prathyushi Nangia <prathyushi.nangia@intel.com>
Reviewed-by: Matthew S Atwood <matthew.s.atwood@intel.corp-partner.google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[add] https://crrev.com/be7e0f36733e822bf7df90fcfc60f0cf9485e44c/dev-db/mariadb/files/mariadb-5.5.32-sql-bison-compatibility.patch
[rename] https://crrev.com/be7e0f36733e822bf7df90fcfc60f0cf9485e44c/dev-db/mariadb/mariadb-5.5.32-r11.ebuild
[modify] https://crrev.com/be7e0f36733e822bf7df90fcfc60f0cf9485e44c/dev-db/mariadb/mariadb-5.5.32.ebuild