[libcxxabi] Make test tolerant of uncommon floating literal demanglings

libcxxabi uses the C99 library's %a format specifier to turn a floating
point value into a hexadecimal string representation. The %a format
specifier is rather loosely defined in the C spec though, and some C
libraries emit a different (but valid) string for a given floating
point value. In particular, the C spec only requires that there is a
single, non-zero hexadecimal digit on the left of the decimal point.
Given that constraint, there are typically four different valid 
representations of a floating point number. I have updated the test to
accept any of the valid representations for demangled floating
point literals.

llvm-svn: 258311