Makefile: Add build time and compiler info string

When we are doing opensbi development, we want to know the build time
and compiler info for debug purpose.

To enable this message, please add "BUILD_INFO=y", like:

```
make BUILD_INFO=y
```

NOTE: Using `BUILD_INFO=y` without specifying SOURCE_DATE_EPOCH will
violate "reproducible builds". So it's ONLY for development and debug
purpose, and should NOT be used in a product which follows "reproducible
builds".

Signed-off-by: Wei Fu <wefu@redhat.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>