MIPS64: Fix MIPS64 related issues in GN files

Sets sysroot and few missing files for MIPS64 Android.

BUG=sysroot is an empty string
TEST=gn gen out-gn/mips64-android --args="is_debug=false target_os=\"android\" target_cpu=\"mips64el\" chrome_public_apk_use_chromium_linker=false"
     ninja -C out-gn/mips64-android system_webview_apk chrome_public_apk

Review-Url: https://codereview.chromium.org/1984303002
Cr-Original-Commit-Position: refs/heads/master@{#394379}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: be0e03709bbdb62d6c8585c2a191502a41ea756f

config/sysroot.gni

@@ -29,7 +29,7 @@ if (current_os == target_os && current_cpu == target_cpu &&
     sysroot = "$android_ndk_root/$x86_64_android_sysroot_subdir"
   } else if (current_cpu == "arm64") {
     sysroot = "$android_ndk_root/$arm64_android_sysroot_subdir"
-  } else if (current_cpu == "mips64") {
+  } else if (current_cpu == "mips64el") {
     sysroot = "$android_ndk_root/$mips64_android_sysroot_subdir"
   } else {
     sysroot = ""

toolchain/android/BUILD.gn

@@ -110,5 +110,5 @@ android_gcc_toolchains_helper("mips64el") {
   android_ndk_sysroot = "$android_ndk_root/$mips64_android_sysroot_subdir"
   android_ndk_lib_dir = "usr/lib64"
 
-  toolchain_cpu = "mipsel64el"
+  toolchain_cpu = "mips64el"
 }