Include translation unit filename in global ctor symbol names.

This makes it easier to see where a global ctor comes from, and it also makes
ASan's init order analyzer output easier to understand.  gcc does this too,
but only in -fPIC mode for some reason.  Don't do this for constructors with
explicit init priority.

Also prepend "sub_" before the 'I', that way regular constructors stay
lexicographically after symbols with init priority (because
ord('s') > ord('I')).  gold seems to ignore the name of constructor symbols,
and ld only looks at the symbol if it includes an init priority, which this
patch doesn't change.

Before: __GLOBAL_I_a
Now: __GLOBAL_sub_I_myfile.cc