DynaLoader shouldn't use mod2fname when finding .bs files.

DynaLoader shouldn't use mod2fname when finding .bs files.

Some platforms (probably only VMS and Android at the moment) take
special steps via the function DynaLoader::mod2fname to construct
a dynamic library name that will be unique and (if too long)
truncated.  Then DynaLoader looks for a bootstrap file with the
exact same name as the dynamic library except with a .bs file
extension.

However, ExtUtils::MakeMaker has never produced bootstrap files
that have been run through mod2fname, so while a Foo:Bar extension
would produce a loadable library named PL__Foo_Bar.exe, the
bootstrap would be called Bar.bs. That shouldn't be a problem
since the bootstrap file is just Perl code read by Perl, but
DynaLoader has (apparently forever) been looking for
PL__Foo_Bar.bs and not finding it. So let's look for it by the
name under which it actually exists.

There are no core extensions that produce non-empty bootstrap
files and no existing test coverage, but as-yet-unintegrated
versions of MakeMaker do have such tests.  See, for example,

https://github.com/Perl-Toolchain-Gang/ExtUtils-MakeMaker/commit/7f5e9a35addeea7ebfcded28277c85f723e1a5de