BaseTools: Add external definitions for RISC-V assembly build

BaseTools: Add external definitions for RISC-V assembly build

Add opensbi external definitions to RISC-V build for assembly
code. Use GCC5_RISCV_OPENSBI_TYPES to refer to edk2 data type
for assembly files instead of using opensbi data type.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>