Suport gcc7

[zoumaguanxin]

src/core/lib/gpr/murmur_hash.cc: In function ‘uint32_t gpr_murmur_hash3(const void*, size_t, uint32_t)’: src/core/lib/gpr/murmur_hash.cc:63:10: error: this statement may fall through [-Werror=implicit-fallthrough=] k1 ^= (static_cast<uint32_t>(keyptr[2])) << 16; ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ src/core/lib/gpr/murmur_hash.cc:65:5: note: here case 2: ^~~~ src/core/lib/gpr/murmur_hash.cc:66:10: error: this statement may fall through [-Werror=implicit-fallthrough=] k1 ^= (static_cast<uint32_t>(keyptr[1])) << 8; ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ src/core/lib/gpr/murmur_hash.cc:68:5: note: here case 1: ^~~~ cc1plus: all warnings being treated as errors make[3]: *** [/home/data/projects/cloudminds_cd_gitlab/server_ws/build/grpc/grpc_build/objs/opt/src/core/lib/gpr/murmur_hash.o] Error 1 make[3]: *** waitting .... third_party/cares/cares/ares_data.c: In function ‘ares_malloc_data’: third_party/cares/cares/ares_data.c:167:40: error: this statement may fall through [-Werror=implicit-fallthrough=] ptr->data.txt_ext.record_start = 0; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~ third_party/cares/cares/ares_data.c:170:7: note: here case ARES_DATATYPE_TXT_REPLY: ^~~~ third_party/cares/cares/ares_data.c: At top level: cc1: error: unrecognized command line option ‘-Wno-invalid-source-encoding’ [-Werror] cc1: all warnings being treated as errors