amd64-linux revamp for compressing shared libraries

Major changes: p_lx_elf.cpp and stub/src/amd64-linux.elf-so*.
(Includes run-time infrastructure for multiple de-compressors.)
	modified:   linker.cpp
	modified:   p_lx_elf.cpp
	modified:   p_lx_elf.h
	modified:   p_unix.cpp
	modified:   p_unix.h
	modified:   stub/Makefile
	modified:   stub/src/amd64-darwin.dylib-entry.S
	modified:   stub/src/amd64-darwin.macho-entry.S
	new file:   stub/src/amd64-expand.S
	modified:   stub/src/amd64-linux.elf-entry.S
	new file:   stub/src/amd64-linux.elf-so_entry.S
	new file:   stub/src/amd64-linux.elf-so_fold.S
	new file:   stub/src/amd64-linux.elf-so_fold.lds
	new file:   stub/src/amd64-linux.elf-so_main.c
	modified:   stub/src/amd64-linux.shlib-init.S
	modified:   stub/src/amd64-win64.pep.S
	modified:   stub/src/arch/amd64/nrv2b_d.S
	modified:   stub/src/arch/amd64/nrv2d_d.S
	modified:   stub/src/arch/amd64/nrv2e_d.S
           plus many generated stub/*.h and stub/tmp/*.bin.dump

src/p_lx_elf.h

@@ -61,7 +61,7 @@ protected:
         unsigned const brka
     ) = 0;
     virtual void defineSymbols(Filter const *);
-    virtual void addStubEntrySections(Filter const *);
+    virtual void addStubEntrySections(Filter const *, unsigned m_decompr);
     virtual void unpack(OutputFile *fo) override;
     unsigned old_data_off, old_data_len;  // un_shlib
 
@@ -568,7 +568,7 @@ protected:
     virtual void pack1(OutputFile *, Filter &) override;  // generate executable header
 
     virtual void buildLoader(const Filter *) override;
-    virtual void addStubEntrySections(Filter const *) override;
+    virtual void addStubEntrySections(Filter const *, unsigned m_decompr) override;
     virtual Linker* newLinker() const override;
     virtual void defineSymbols(Filter const *) override;
 };