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

Apply this in add_compiler_rt_runtime instead of manually adding it
to the individual projects. This applies the option on more
parts of compiler-rt than before, but should ideally not make any
difference assuming the other runtimes that lacked the option
also were C11 compatible.

Not marking this as required, to match the existing behaviour (where
`-std=c11` was added only if supported by the compiler).

This was suggested during the review of D110005.

Differential Revision: https://reviews.llvm.org/D124343