[riscv64][android] support 64-bit builds on riscv64

For arch riscv, android supports riscv64 only.
So, only riscv64 is added to be supported.

co-authors: McKnight22 <tao.wang.2261@gmail.com>
            Wang Chen <wangchen20@iscas.ac.cn>

Bug: angleproject:8165
Change-Id: Iaa732ece963e82abeb7ae517eae3a26e1314f56c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4530628
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>

AUTHORS

@@ -41,6 +41,7 @@ The Khronos Group, Inc.
 Numfum GmbH
 Yandex LLC
 Rive
+Institute of Software, Chinese Academy of Sciences
 
 Jacek Caban
 Mark Callow
@@ -79,3 +80,4 @@ SeongHwan Park
 Xiaopeng Li
 Akihiko Odaki
 Ho Cheung
+Tao Wang

CONTRIBUTORS

@@ -232,3 +232,6 @@ Yandex LLC
 
 Rive
  Chris Dalton
+
+Institute of Software, Chinese Academy of Sciences
+ Wang Chen

gni/angle.gni

@@ -107,7 +107,8 @@ declare_args() {
 
   if (current_cpu == "arm64" || current_cpu == "x64" ||
       current_cpu == "mips64el" || current_cpu == "s390x" ||
-      current_cpu == "ppc64" || current_cpu == "loong64") {
+      current_cpu == "ppc64" || current_cpu == "loong64" ||
+      current_cpu == "riscv64") {
     angle_64bit_current_cpu = true
   } else if (current_cpu == "arm" || current_cpu == "x86" ||
              current_cpu == "mipsel" || current_cpu == "s390" ||