Add board name for emulator

VTS checks for ro.product.board before running. Emulator does not have
that value and causes an exception.

So let's add it to the emulator and call it goldfish_$(TARGET_ARCH).

BUG: 73741117
Test: vts-tradefed run vts, should run the tests
Change-Id: I6b00f2923bc9609d4d05c45d47ceddd2bd7be091