Fix fyne release --os android not respecting ldflags present in GOFLAGS environment variable

[petoem]

Fixes #69

[sdassow]

Without having worked in that particular area of the code, it reminds me of #17.

[petoem]

Looked at it again. #17 fixes windows builds by not changing the environment, but that's different for android.

I explain more about this PR:

• To make the handling of ldflags more consistent, I moved the functions for extracting them to the util package. In the process of doing this I fixed the same bug as Fix ldflags in GOFLAGS not being respected. #17 . (have not tested it)
• To fix mobile build, I had to set the buildLdflags and pass the remaining GOFLAGS on to the go build command. Here I used the same ExtractLdFlags function mentioned above.

This pull request probably fixes ldflags for both mobile and desktop.
It would be great if someone could test if it truly fixes windows builds too.