Revert "Actually build libc++ and libc++abi with -std=c++20"

This reverts commit d95d20a3f7cc630e324e706c0a2bd405792283c4.

Reason for revert: speculative for https://crbug.com/1298543

Original change's description:
> Actually build libc++ and libc++abi with -std=c++20
>
> build/config/compiler/BUILD.gn adds the flag to cflags_cc, so this
> config here must do that too. Else the -std=c++20 will show up
> on the commandline _before_ the -std=c++17 added by
> build/config/compiler:compiler, rendering it ineffective.
>
> Bug: 1298070
> Change-Id: I145b04aeef1003620594a0be8f74bb7731797069
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3469402
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Reviewed-by: Arthur Eubanks <aeubanks@google.com>
> Commit-Queue: Arthur Eubanks <aeubanks@google.com>
> Cr-Commit-Position: refs/heads/main@{#972250}

Bug: 1298070
Change-Id: Id6d8d757e0812ff79762627db3ccbb8aa1e10576
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3472585
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Robert Kroeger <rjkroege@google.com>
Commit-Queue: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/main@{#972734}
NOKEYCHECK=True
GitOrigin-RevId: 4f636b9538f7b33a8a4d41790cfb565d132add59
1 file changed