lib/build_OID_registry: avoid non-destructive substitution for Perl < 5.13.2 compat

lib/build_OID_registry: avoid non-destructive substitution for Perl < 5.13.2 compat

On a system with Perl 5.12.1, commit 5ef6dc08cfde
("lib/build_OID_registry: don't mention the full path of the script in
output") causes the build to fail with the error below.

     Bareword found where operator expected at ./lib/build_OID_registry line 41, near "s#^\Q$abs_srctree/\E##r"
     syntax error at ./lib/build_OID_registry line 41, near "s#^\Q$abs_srctree/\E##r"
     Execution of ./lib/build_OID_registry aborted due to compilation errors.
     make[3]: *** [lib/Makefile:352: lib/oid_registry_data.c] Error 255

Ahmad Fatoum analyzed that non-destructive substitution is only supported since
Perl 5.13.2. Instead of dropping `r` and having the side effect of modifying
`$0`, introduce a dedicated variable to support older Perl versions.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 5ef6dc08cfde ("lib/build_OID_registry: don't mention the full path of the script in output")
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Paul Menzel <[email protected]>
Suggested-by: Ahmad Fatoum <[email protected]>
Cc: Uwe Kleine-König <[email protected]>
Cc: Nicolas Schier <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Ahmad Fatoum <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>