package/go: bump version to 1.16

package/go: bump version to 1.16

Release notes: https://golang.org/doc/go1.16

The latest Go release, version 1.16, arrives six months after Go 1.15. Most of
its changes are in the implementation of the toolchain, runtime, and libraries.

The linker changes in 1.16 extend the 1.15 improvements to all supported
architecture/OS combinations (the 1.15 performance improvements were primarily
focused on ELF-based OSes and amd64 architectures). For a representative set of
large Go programs, linking is 20-25% faster than 1.15 and requires 5-15% less
memory on average for linux/amd64, with larger improvements for other
architectures and OSes. Most binaries are also smaller as a result of more
aggressive symbol pruning.

According to the release notes, Go 1.16 drops support for x87 mode
compilation (GO386=387).  Support for non-SSE2 processors is now available
using soft float mode.  Buildroot will automatically set GO386=softfloat on
non-SSE2 processors.

Signed-off-by: Christian Stewart <christian@paral.in>
v1 -> v2:

 - added 386=softfloat handling re: Peter's review

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>