BaseTools/tools_def RISCV: Make OpenSBI references RISCV-only

BaseTools/tools_def RISCV: Make OpenSBI references RISCV-only

The global GCC_PP_FLAGS tools_def variable now contains a reference to
OpenSBI specific C preprocessor variables, which means they are added to
the command line on every architecture, not just RISC-V.

This does not currently result in any issues, but it is a bit sloppy so
let's clean this up. Given that the GCC_PP_FLAGS definition appears
twice, drop the one that carries the OpenSBI reference, and move that
reference to a new RISC-V specific variable.

Acked-by: Abner Chang <abner.chang@amd.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>