Fixes for ARM 32/64

[julien-lebot]

This PR introduces a couple of fixes for ARM32/64:

• Conditionally set GIMME_ARCH so that go builds don't fail on 64 bit ARM builds
• Update entrypoint.sh to use GIMME_ARCH for ARM builds so that eval "$(gimme)" finds the correct go binary folder
• Use fakearmv7l LD_PRELOAD library to patch uname at runtime to return architecture armv7l for RPM 32 bit build

[mx-psi]

LGTM, just a typo

[mx-psi]

LGTM, left a couple of nits but feel free to ignore them

[mx-psi]

Why did we change the base image? (just for future reference)

[julien-lebot]

Initially it was amazonlinux, see #21. Amazon Linux however, doesn't provide 32 bit versions of their image, so we switched to CentOS 7 (which is what Amazon Linux is supposed to be based on I think). However, CentOS 7 is quite old and ships with GCC 4.8.5 - which causes issues when trying to build rtloader.
Hence we had two options:

• Install a newer GCC on CentOS 7
• Revert to the Amazon Linux image for 64 bit builds

We chose the latter because we don't build rtloader for 32 bit ARM, so it's simpler to revert to the 'known to work' image.

[mx-psi]

Could we use Python 3 here? Base image doesn't have it so it needs to be installed. Not a blocker but it would useful if we ever need a Python 3 only package.

[julien-lebot]

Good point, I think we could have python 3 in this image, but I think this should be addressed as a separate PR