[Hexagon] Remove std::hex in favor of format().

std::hex is not used anywhere in LLVM code base except for this place,
and it has a known undefined behavior (at least in libstdc++ 4.9.3):
https://llvm.org/bugs/show_bug.cgi?id=18156, which fires in UBSan
bootstrap of LLVM.

llvm-svn: 254547