libcxxrt: fix demangling of wchar_t

'wchar_t' is 7 characters long, not 6. r303297 fixed this in libelftc,
but not the second copy of this file that we have in libcxxrt.

PR: 208661
Submitted by: Daniel McRobb
Obtained from: ELF Tool Chain r3480
MFC after: 3 days