libxml2, libxslt: upgraded packages to upstream

Upgraded dev-libs/libxml2 to version 2.9.12 on amd64, arm
Upgraded dev-libs/libxslt to version 1.1.34-r1 on amd64, arm

Additional changes included:
-Include python 3.6 support in libxml2 ebuild
-Bump libxslt from r1 (upstream) to r3 as existing chromium version was
r2.

BUG=1211312
TEST=Locally run arm64-generic in VM + trybots

Change-Id: Ibf24b2f6b32b1603818af6d635a0c1470d4d3344
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2914237
Commit-Queue: Allen Webb <allenwebb@google.com>
Reviewed-by: Allen Webb <allenwebb@google.com>
Tested-by: Allen Webb <allenwebb@google.com>
(cherry picked from commit b0e49456c1ad20ce80e9d6e567ee12386afa2073)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/3043487
Commit-Queue: Daniel Gagnon <dgagnon@google.com>
Tested-by: Daniel Gagnon <dgagnon@google.com>
21 files changed