DMSGoPGP: Support Bitcode

[jk234ert]

Refer to discussion here, currently DMSGoPGP has several technical issues:

1. DMSGoPGP supports Bitcode
2. Get rid of symbolizing error in submitting to AppStore

To resolve, need to try the following steps as shared by community:

1. go get -u golang.org/x/mobile/cmd/... to fetch latest gomobile version
2. Build gomobile framework with -ldflags=-w
3. Enable Bitcode and 'Upload symbols to AppStore' to check if there still exists errors.

[jk234ert]

Should be fixed in latest TestFlight release 1.1.5(26) DMSGoPGP