[Committed] Fix bootstrap on x86_64 with recent build_string change

[Jason Merrill]

I ran into this problem too, and came up with the same fix.

While I was investigating it, I noticed that callers of build_string are 
very inconsistent about whether or not the length parameter should 
include the terminal NUL.  It seems that historically it was supposed 
to, but this isn't documented, many callers don't include it, 
build_string doesn't expect it, and it seems surprising for 
TREE_STRING_LENGTH to include the terminal NUL while IDENTIFIER_LENGTH 
doesn't.  I think we ought to document that TREE_STRING_LENGTH does 
*not* include the terminal NUL and unify the users accordingly.

Jason