Revert "Put the win host system-allocator toolchain actually on the host cpu"

This reverts commit 10bd93d45d656ba125269b32a1fec49fdc53dc94.

Reason for revert: official/win-clang is still broken by:
https://crrev.com/c/5645742
failing build:
https://ci.chromium.org/ui/p/chrome/builders/official/win-clang/8651/overview

Original change's description:
> Put the win host system-allocator toolchain actually on the host cpu
>
> Like the rust host build tools toolchain, get the system-allocator
> toolchain to use the actual host cpu type. This was intended but
> overlooked in https://crrev.com/c/5645742, which removed the hacks
> that defined the symupload toolchain name directly to get to the
> real host cpu.
>
> R=hans@chromium.org
>
> Bug: 349268750
> Change-Id: I2e89c8dfb73ba53f9bb40254241d2a5ad68fa426
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5650284
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Hans Wennborg <hans@chromium.org>
> Reviewed-by: Hans Wennborg <hans@chromium.org>
> Auto-Submit: danakj <danakj@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1319148}

Bug: 349268750
Change-Id: I3887c863500f9a4f0e11d71ea3c21f123b5add63
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5656391
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Will Yeager <wyeager@google.com>
Reviewed-by: Keybo Qian <keybo@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#1319534}

• build/toolchain/win/toolchain.gni[diff]