[superproject] Roll llvm-project/libc [libc][docs] disable pthreads docs (#123824)

Having a target named pthreads is breaking when multiple runtimes are enabled.
Disable this target for now so that the builds go back to green (and sites get
updated).

Link: llvm/llvm-zorg#359 (comment)
Link: #122006
Link: #122497
Link: #123821
GitOrigin-RevId: 6ac1fb4fd88250e38b9c0c7d9af792cb82c2da05
Original-Revision: e58422d81dea98c3acd2b044916d905ec13ca781
Roller-URL: https://cr-buildbucket.appspot.com/build/8725001939711279313
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I2690cc26b8ec6877812e13a72a87ff7df2559d9f
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1191992

Author: global-integration-roller