ANDROID: GKI: Add 64-bit RISC-V config

Add a "fake" GKI for RISC-V based on the arm64 one. This implementation
simply symlinks the arm64 gki_defconfig and merges in two other configs:

- arch/riscv/configs/64-bit.config
  This is an upstream file which enables RISC-V for 64-bit.

- arch/riscv/configs/gki.config
  This is a workaround file which enables things the ARM64 architecture
  pre-selects, such as GPIOLIB. Without enabling these options, some
  drivers are incorrectly dropped from the gki_defconfig.

Bug: 260012551
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: Ifad59ed483bdbb736e905eefc5107328155a361b