GN supports 32-bits LittleEndian targets (mipsel), but GN does not supports 64-bits mips64el. 

https://chromium-review.googlesource.com/c/566340/

I think support for big endian landed already (associated to some other bug). So the only remaining thing is to support mips64el? Maybe change this CL desc?

I new  issue 740468 

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/ae1fbc0dc502ffab2b8c0f4715562f8c8e847920

commit ae1fbc0dc502ffab2b8c0f4715562f8c8e847920
Author: Michael Achenbach <machenbach@chromium.org>
Date: Wed Jan 31 14:38:43 2018

[build] Enable downloading mips toolchain via runhooks

This downloads the mips tool chain into tools/mips_toolchain via
runhooks if the flag download_mips_toolchain is specified for
gclient.

The currently uploaded version is 2015/1/7.

Bug:  chromium:632390 
Change-Id: Ied396d1a980db962773a96e9dadd3f282c7541c0
Reviewed-on: https://chromium-review.googlesource.com/892864
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50999}
[modify] https://crrev.com/ae1fbc0dc502ffab2b8c0f4715562f8c8e847920/.gitignore
[modify] https://crrev.com/ae1fbc0dc502ffab2b8c0f4715562f8c8e847920/DEPS
[add] https://crrev.com/ae1fbc0dc502ffab2b8c0f4715562f8c8e847920/tools/mips_toolchain.tar.gz.sha1

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/a1b2e2a754d1ddc502882e7a5fb726aab07b3a39

commit a1b2e2a754d1ddc502882e7a5fb726aab07b3a39
Author: Michael Achenbach <machenbach@chromium.org>
Date: Wed Jan 31 15:00:49 2018

V8: Enable mips builder to download bundled toolchain

Depends on: https://crrev.com/c/892864

TBR=sergiyb@chromium.org

Bug:  chromium:632390 
Change-Id: If2d831625d90651cdbe2dd93687f40b133fd8e61
Reviewed-on: https://chromium-review.googlesource.com/893558
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>

[modify] https://crrev.com/a1b2e2a754d1ddc502882e7a5fb726aab07b3a39/scripts/slave/recipe_modules/v8/builders.py
[modify] https://crrev.com/a1b2e2a754d1ddc502882e7a5fb726aab07b3a39/scripts/slave/recipe_modules/v8/gclient_config.py
[modify] https://crrev.com/a1b2e2a754d1ddc502882e7a5fb726aab07b3a39/scripts/slave/recipes/v8.expected/full_client_v8_ports_V8_Mips___builder.json

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/822774b78dd8b7ee59854d34ccc4bf2eaff2662c

commit 822774b78dd8b7ee59854d34ccc4bf2eaff2662c
Author: Michael Achenbach <machenbach@chromium.org>
Date: Wed Jan 31 18:31:50 2018

V8: Use bundled mips toolchain

This is now possible after:  https://crrev.com/c/892864

TBR=sergiyb@chromium.org

Bug:  chromium:632390 
Change-Id: I4e9cd2e3f0201005db8f97780e9501f9f3fbf7ca
Reviewed-on: https://chromium-review.googlesource.com/894527
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>

[modify] https://crrev.com/822774b78dd8b7ee59854d34ccc4bf2eaff2662c/scripts/slave/README.recipes.md
[modify] https://crrev.com/822774b78dd8b7ee59854d34ccc4bf2eaff2662c/scripts/slave/recipe_modules/v8/api.py
[modify] https://crrev.com/822774b78dd8b7ee59854d34ccc4bf2eaff2662c/scripts/slave/recipes/v8.expected/full_client_v8_ports_V8_Mips___builder.json

We can now build mips (BE) with GN. Manually verified that the builds work on our mips boards (we have always built without snapshot and without i18n support).

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/8f7ccb6c1e7fb321d280c405b8893838c8bbadab

commit 8f7ccb6c1e7fb321d280c405b8893838c8bbadab
Author: Michael Achenbach <machenbach@chromium.org>
Date: Thu Feb 08 08:25:47 2018

[build] Support mips toolchain for cross-compiling

This enables compiling V8 stand-alone for mips (big endian)
on an ia32 host with gn args:

custom_toolchain="//:mips-bundled"
v8_use_snapshot=false
v8_enable_i18n_support=false
icu_use_data_file=false
is_debug=false
target_cpu="mips"

Bug:  chromium:632390 
Change-Id: I0b59126d742a2979dd0fcbcdd3a574008a09479a
Reviewed-on: https://chromium-review.googlesource.com/904990
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51167}
[add] https://crrev.com/8f7ccb6c1e7fb321d280c405b8893838c8bbadab/tools/toolchain/BUILD.gn