[sairpcgen][ENH] Add way to link additional libraries to compiled rpcserver

[chrispsommers]

Some libsai implementations require additional libraries besides libsai.so to link the final target. There needs to be a way to pass instructions to the final linker phase in the Makefile, e.g. see code below. I will follow-up with a PR to add this.

# Define extra libs for rpcserver:
SAIRPC_EXTRA_LIBS="\
		-L/lib/x86_64-linux-gnu -Wl,-rpath=/lib/x86_64-linux-gnu -lm \
		-L/usr/local/lib/ -Wl,-rpath=/usr/local/lib \
	        -lpthread \
	        -lpiprotogrpc \
	        -lpiprotobuf \
	        -lprotobuf \
	        -lgrpc++ \
	        -lgrpc \
	        -lpiall \
	        -lpi_dummy \
	        -lpthread \
	        -labsl_synchronization \
	        -labsl_status \
	        -labsl_raw_hash_set \
		-lgpr \
		-lre2 \
		-lssl \
		-laddress_sorting"

# Build the rpcserver:
export SAITHRIFTV2=y && \
export GEN_SAIRPC_OPTS="-ve" && \
export SAIRPC_EXTRA_LIBS=$(SAIRPC_EXTRA_LIBS) && \
make saithrift-build

[chrispsommers]

Fixed by #1533