Place loader immediately after compressed PT_LOADs, in "middle" of file.

This moves compressed gaps and debuginfo out of compressed PT_LOADs,
and fixes a bug where large debuginfo would cause first mmap
at runtime to erase the loader (SourceForge bug 3239079).

src/p_lx_elf.h

@@ -75,6 +75,8 @@ protected:
     unsigned sz_phdrs;  // sizeof Phdr[]
     unsigned sz_elf_hdrs;  // all Elf headers
     unsigned sz_pack2;  // after pack2(), before loader
+    unsigned sz_pack2a;  // after pack2() of all PT_LOAD
+    unsigned sz_pack2b;  // after pack2() of all PT_LOAD and gaps
     unsigned lg2_page;  // log2(PAGE_SIZE)
     unsigned page_size;  // 1u<<lg2_page
     unsigned xct_off;  // shared library: file offset of SHT_EXECINSTR