ansible: build gn from source on Linux PPC/s390x

[richardlau]

Add build-test-v8 meta-role for installing prerequisites for V8
building/testing. Add roles to build gn and ninja binaries from
source and call those from the build-test-v8 role for Linux on
ppc64/s390x.

This removes a previously manual step from the Linux PPC/s390x setup (docs also updated).

I've deployed this onto

• test-ibm-rhel7-s390x-1
• test-ibm-rhel7-s390x-2
• test-ibm-rhel7-s390x-3
• test-ibm-rhel7-s390x-4
• test-osuosl-centos7-ppc64_le-1
• test-osuosl-centos7-ppc64_le-2

to update the version of gn (the version we'd previously manually compiled is now too old, see nodejs/node#38990 (comment)).
We're now updated to:

[iojs@test-osuosl-centos7-ppc64--le-1 build-tools]$ ./ninja --version
1.10.2
[iojs@test-osuosl-centos7-ppc64--le-1 build-tools]$ ./gn --version
1916 (d2dce75)
[iojs@test-osuosl-centos7-ppc64--le-1 build-tools]$

[mhdawson]

LGTM