[libcxx] Mark stray symbols as hidden to try and fix the build

r347395 changed the ABI list on Linux, but two of those symbols are still
being exported from the shared object:

    _ZSt18make_exception_ptrINSt3__112future_errorEESt13exception_ptrT_
    _ZNSt3__1plIcNS_11char_traitsIcEENS_9allocatorIcEEEENS_12basic_stringIT_T0_T1_EERKS9_PKS6_

This commit makes sure those symbols are not exported, as they should be.

llvm-svn: 347399