build on ARM64 (facebook#5450)

Summary:
Support building RocksDB on AWS ARM64

```
uname -m
aarch64
```
Pull Request resolved: facebook#5450

Differential Revision: D15879851

fbshipit-source-id: a9b56520a2cd9921338305a06d7103a40a3300b8

build_tools/build_detect_platform

@@ -537,7 +537,7 @@ if test -z "$PORTABLE"; then
     COMMON_FLAGS="$COMMON_FLAGS -mcpu=$POWER -mtune=$POWER "
   elif test -n "`echo $TARGET_ARCHITECTURE | grep ^s390x`"; then
     COMMON_FLAGS="$COMMON_FLAGS -march=z10 "
-  elif test -n "`echo $TARGET_ARCHITECTURE | grep ^arm`"; then
+  elif test -n "`echo $TARGET_ARCHITECTURE | grep -e^arm -e^aarch64`"; then
     # TODO: Handle this with approprite options.
     COMMON_FLAGS="$COMMON_FLAGS"
   elif test -n "`echo $TARGET_ARCHITECTURE | grep ^aarch64`"; then