Revert "[compiler-rt] Use C_STANDARD instead of custom logic for adding -std=c11"

This reverts commit 2f251925.

This was reported to break the Builtins-powerpc64le-linux::qdiv_test.c
test on ppc64le buildbots, e.g. these:

https://lab.llvm.org/buildbot/#/builders/105/builds/24538
https://lab.llvm.org/buildbot/#/builders/121/builds/18755