[compiler-rt] Clean up explicit -std=c++17 addition

This was superseded by 5737f6a527d782e6577e5cc1e0c743df2c98546a. Clean
it up as suggested by Mike Hommey in https://reviews.llvm.org/D131937.

GitOrigin-RevId: d7a4ff9986c8dcaf967770c868e9020d3f328e9f
2 files changed