BaseTools: Add DTCPP_FLAGS for GCC5 RISCV64 toolchain

BaseTools: Add DTCPP_FLAGS for GCC5 RISCV64 toolchain

Some/all platforms are going to require EDK2 to build a device tree and
use it in the early stages of boot.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Abner Chang <abner.chang@hpe.com>
Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>