riscv: -fPIC for c++ apps

mk/vars.mk

@@ -201,6 +201,7 @@ endif
 ifeq ($(ADK_TARGET_ARCH_RISCV64)$(ADK_TARGET_ARCH_RISCV32),y)
 ifeq ($(ADK_TARGET_BINFMT_FLAT),y)
 TARGET_CFLAGS+=		-fPIC
+TARGET_CXXFLAGS+=	-fPIC
 endif
 endif