Occlum-compatible executable binaries must be Position-Independent
Executable (PIE). Previously, to build such binaries, the users need to
explicitly give `-buildmode=pie` flag to `occlum-go`. Apparently, this
is error-prone. This commit sets `-buildmode=pie` by default for `occlum-go`.
In addition, this commit upgrades the Go version to 1.13.7.
This commit provides a modified Go runtime in Docker image.
Now we can build a Go program using `occlum-go`, then run it
in SGX enclaves by Occlum.
The Golang demo demonstrates how to build and run a web server
program written in Go.
This commit makes the toolchain easier to use in two folds:
1. When compiling C/C++ source files, no need to add "-fPIC -pie" flags manually;
2. When running executables generated by the Occlum toolchain on Linux, no
need to set the `LD_LIBRARY_PATH` manually.
