Add ability to disable bitcode

[jeanregisser]

Description

This PR adds a new DISABLE_BITCODE environment variable to be able to disable bitcode.

Why

Currently bitcode embedding is enabled by default for go >= 1.14 and there was no way to disable it.

In our celo-org/celo-blockchain project (written in Go) we currently need bitcode disabled because one of the dependencies is written in Rust and we've had compatibility issues with the Rust generated bitcode and recent version of Xcode.

We were good with go 1.13 (bitcode is disabled there), but we wanted to upgrade to go 1.14 and got the following error:

go build runtime/cgo: invalid flag in go:cgo_ldflag: -fembed-bitcode

See more in https://app.circleci.com/pipelines/github/celo-org/celo-blockchain/4999/workflows/9807228b-1b99-44f2-90f7-371bfeba9e97/jobs/46791

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[ydnar]

@jeanregisser could you try #70 and see if it still has the issue? ddd6497b067577b19dddcb2b88017887a0227eb5 enabled bitcode unconditionally, but I’m curious if this issue with Rust still exists.

[jeanregisser]

Dropping this, as bitcode is deprecated since Xcode 14.