internal/rosa: remove legacy exec helper

This change also adds a fourth stage to the bootstrap machinery, offering more correct toolchain validation that works around LLVM dynamic linking flaws.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/perl.go

@@ -9,7 +9,7 @@ type MakeMakerHelper struct {
 }
 
 // extra returns perl.
-func (*MakeMakerHelper) extra(int) P { return P{_perl, _make} }
+func (*MakeMakerHelper) extra() P { return P{_perl, _make} }
 
 // wantsChmod returns true.
 func (*MakeMakerHelper) wantsChmod() bool { return true }