RISC-V: Adjust build rule for gen-riscv-ext-opt and gen-riscv-ext-texi

RISC-V: Adjust build rule for gen-riscv-ext-opt and gen-riscv-ext-texi

Separate the build rules to compile and link stage to make sure
BUILD_LINKERFLAGS and BUILD_LDFLAGS are applied correctly.

We hit this issue when we try to build GCC with non-system-default g++,
and it use newer libstdc++, and then got error from using older libstdc++ from
system, that should not happened if we link with -static-libgcc and
-static-libstdc++.

gcc/ChangeLog:

	* config/riscv/t-riscv: Adjust build rule for gen-riscv-ext-opt
	and gen-riscv-ext-texi.