Build libc++ as C++26

Matches upstream: https://github.com/llvm/llvm-project/pull/181021

We rolled that in in
https://chromium-review.googlesource.com/c/chromium/src/+/7789956

(See upstream b763d6a4ed465 for c++latest mapping to C++26.)

(Also move the `-fPIC` flag out of the language version setting block.)

Bug: 503250165
Change-Id: I3f5ef7a87d817726bb4525f24357277ff4139fc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7800621
Commit-Queue: Hans Wennborg <hans@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1622276}
NOKEYCHECK=True
GitOrigin-RevId: cd9dea17202e00ad4e9a3a6a5d38d9a4bcd3477e