Restore android/amd64 target

Underlying syscall.Pipe issue was fixed in https://golang.org/cl/350530
and https://golang.org/cl/350549.

MobileLibrary/Android/make.bash

@@ -72,23 +72,7 @@ echo " Gomobile version: ${GOMOBILEVERSION}"
 echo " Dependencies: ${DEPENDENCIES}"
 echo ""
 
-# Note: android/386 is x86, which is used on both x86 and x86_64 Android
-# devices. We are excluding the android/amd64, x86_64, ABI as it causes a
-# crash in Android x86_64 emulators: "seccomp prevented call to disallowed
-# x86_64 system call 22". x86/linux syscall 22 is pipe.
-#
-# In Android seccomp config, pipe is permitted only for 32-bit platforms:
-# https://android.googlesource.com/platform/bionic/+/2b499046f10487802bfbaaf4429160595d08b22c/libc/SECCOMP_WHITELIST_APP.TXT#7.
-#
-# The Go syscall.Pipe on linux(android)/amd64 is the disallowed pipe:
-# https://github.com/golang/go/blob/release-branch.go1.14/src/syscall/syscall_linux_amd64.go#L115-L126
-#
-# A potential future fix is to use the allowed pipe2,
-# https://android.googlesource.com/platform/bionic/+/2b499046f10487802bfbaaf4429160595d08b22c/libc/SYSCALLS.TXT#129,
-# which is what linux(android)/arm64 uses, for example:
-# https://github.com/golang/go/blob/release-branch.go1.14/src/syscall/syscall_linux_arm64.go#L150-L159.
-
-gomobile bind -v -x -target=android/arm,android/arm64,android/386 -tags="${BUILD_TAGS}" -ldflags="$LDFLAGS" github.com/Psiphon-Labs/psiphon-tunnel-core/MobileLibrary/psi
+gomobile bind -v -x -target=android/arm,android/arm64,android/386,android/amd64 -tags="${BUILD_TAGS}" -ldflags="$LDFLAGS" github.com/Psiphon-Labs/psiphon-tunnel-core/MobileLibrary/psi
 if [ $? != 0 ]; then
   echo "..'gomobile bind' failed, exiting"
   exit $?
@@ -100,6 +84,7 @@ yes | cp -f PsiphonTunnel/AndroidManifest.xml build-tmp/psi/AndroidManifest.xml
 yes | cp -f PsiphonTunnel/libs/armeabi-v7a/libtun2socks.so build-tmp/psi/jni/armeabi-v7a/libtun2socks.so
 yes | cp -f PsiphonTunnel/libs/arm64-v8a/libtun2socks.so build-tmp/psi/jni/arm64-v8a/libtun2socks.so
 yes | cp -f PsiphonTunnel/libs/x86/libtun2socks.so build-tmp/psi/jni/x86/libtun2socks.so
+yes | cp -f PsiphonTunnel/libs/x86_64/libtun2socks.so build-tmp/psi/jni/x86_64/libtun2socks.so
 mkdir -p build-tmp/psi/res/xml
 yes | cp -f PsiphonTunnel/ca_psiphon_psiphontunnel_backup_rules.xml build-tmp/psi/res/xml/ca_psiphon_psiphontunnel_backup_rules.xml