imgact_binmisc: move some calculations out of the exec path
The offset we need to account for in the interpreter string comes in two
variants:
- Fixed - macros other than #a that will not vary from invocation to invocation
- Variable - #a, which is substitued with the argv0 that we're replacing
Note that we don't have a mechanism to modify an existing entry. By
recording both of these offset requirements when the interpreter is added,
we can avoid some unnecessary calculations in the exec path.
Most importantly, we can know up-front whether we need to grab
calculate/grab the the filename for this interpreter. We also get to avoid
walking the string a first time looking for macros. For most invocations,
it's a swift exit as they won't have any, but there's no point entering a
loop and searching for the macro indicator if we already know there will not
be one.
While we're here, go ahead and only calculate the argv0 name length once per
invocation. While it's unlikely that we'll have more than one #a, there's no
reason to recalculate it every time we encounter an #a when it will not
change.
I have not bothered trying to benchmark this at all, because it's arguably a
minor and straightforward/obvious improvement.
MFC after: 1 week