Add support for riscv64 (apache#769)

* Fix riscv64 target_arch

This should be defined for riscv64 instead, as `riscv` doesn't match it.
I have no idea for riscv32 though.

* parquet: Use murmur_hash2_64a for riscv64

Author: felixonmars

arrow/src/alloc/alignment.rs

@@ -62,7 +62,7 @@ pub const ALIGNMENT: usize = 1 << 6;
 // - https://github.com/torvalds/linux/blob/master/Documentation/devicetree/bindings/riscv/sifive-l2-cache.txt#L41
 // in general all of them are the same.
 /// Cache and allocation multiple alignment size
-#[cfg(target_arch = "riscv")]
+#[cfg(target_arch = "riscv64")]
 pub const ALIGNMENT: usize = 1 << 6;
 
 // This size is same across all hardware for this architecture.

parquet/src/util/hash_util.rs

@@ -33,7 +33,7 @@ fn hash_(data: &[u8], seed: u32) -> u32 {
         }
     }
 
-    #[cfg(any(target_arch = "aarch64", target_arch = "arm"))]
+    #[cfg(any(target_arch = "aarch64", target_arch = "arm", target_arch = "riscv64"))]
     unsafe {
         murmur_hash2_64a(data, seed as u64) as u32
     }