[libcxx] Use C++14 when building libc++ with musl

musl's pthread implementations use volatile types in their structs
which is not being constexpr in C++11 but is in C++14.

Differential Revision: https://reviews.llvm.org/D25491

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@284950 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed