Improve build detect for RISCV (#9366)

Summary:
Related to: #9215

* Adds build_detect_platform support for RISCV on Linux (at least on SiFive Unmatched platforms)

This still leaves some linking issues on RISCV remaining (e.g. when building `db_test`):
```
/usr/bin/ld: ./librocksdb_debug.a(memtable.o): in function `__gnu_cxx::new_allocator<char>::deallocate(char*, unsigned long)':
/usr/include/c++/10/ext/new_allocator.h:133: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(memtable.o): in function `std::__atomic_base<bool>::compare_exchange_weak(bool&, bool, std::memory_order, std::memory_order)':
/usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: /usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: /usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: /usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(memtable.o):/usr/include/c++/10/bits/atomic_base.h:464: more undefined references to `__atomic_compare_exchange_1' follow
/usr/bin/ld: ./librocksdb_debug.a(db_impl.o): in function `rocksdb::DBImpl::NewIteratorImpl(rocksdb::ReadOptions const&, rocksdb::ColumnFamilyData*, unsigned long, rocksdb::ReadCallback*, bool, bool)':
/home/adamretter/rocksdb/db/db_impl/db_impl.cc:3019: undefined reference to `__atomic_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(write_thread.o): in function `rocksdb::WriteThread::Writer::CreateMutex()':
/home/adamretter/rocksdb/./db/write_thread.h:205: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(write_thread.o): in function `rocksdb::WriteThread::SetState(rocksdb::WriteThread::Writer*, unsigned char)':
/home/adamretter/rocksdb/db/write_thread.cc:222: undefined reference to `__atomic_compare_exchange_1'
collect2: error: ld returned 1 exit status
make: *** [Makefile:1449: db_test] Error 1
```

Pull Request resolved: #9366

Reviewed By: jay-zhuang

Differential Revision: D34377664

Pulled By: mrambacher

fbshipit-source-id: c86f9d0cd1cb0c18de72b06f1bf5847f23f51118

build_tools/build_detect_platform

@@ -638,6 +638,9 @@ if test "0$PORTABLE" -eq 0; then
       COMMON_FLAGS="$COMMON_FLAGS -march=z196 "
     fi
     COMMON_FLAGS="$COMMON_FLAGS"
+  elif test -n "`echo $TARGET_ARCHITECTURE | grep ^riscv64`"; then
+    RISC_ISA=$(cat /proc/cpuinfo | grep isa | head -1 | cut --delimiter=: -f 2 | cut -b 2-)
+    COMMON_FLAGS="$COMMON_FLAGS -march=${RISC_ISA}"
   elif [ "$TARGET_OS" == "IOS" ]; then
     COMMON_FLAGS="$COMMON_FLAGS"
   elif [ "$TARGET_OS" == "AIX" ] || [ "$TARGET_OS" == "SunOS" ]; then
@@ -658,6 +661,11 @@ else
     COMMON_FLAGS="$COMMON_FLAGS -march=z196 "
   fi
 
+  if test -n "`echo $TARGET_ARCHITECTURE | grep ^riscv64`"; then
+    RISC_ISA=$(cat /proc/cpuinfo | grep isa | head -1 | cut --delimiter=: -f 2 | cut -b 2-)
+    COMMON_FLAGS="$COMMON_FLAGS -march=${RISC_ISA}"
+  fi
+
   if [[ "${PLATFORM}" == "OS_MACOSX" ]]; then
     # For portability compile for macOS 10.12 (2016) or newer
     COMMON_FLAGS="$COMMON_FLAGS -mmacosx-version-min=10.12"