arm64: Use gccgo as bootstrap for compiling golang

The issue is armv6 released binaries are used as a GOROOT_BOOTSTRAP.
This might work on arm64 platforms that support 32-bit mode.
However not all arm64 platforms support 32-bit mode. 32-bit mode
is optional for ARMv8. So use gccgo as bootstrap.

The build image is bumped to use ubuntu wily.

Signed-off-by: Vijaya Kumar K <vijayak@caviumnetworks.com>