build(linux): introduce overlay makefile infrastructure

Replaces the per-version in-tree build files with a shared overlay that
keeps all project-owned build infrastructure outside the vendor source trees,
making it safe to use vendor directories as pristine git submodules.

Changes:
- Add tools/linux/overlay/makefile.gcc: parameterised makefile that drives
  MinGW cross-compilation; VENDOR_7ZIP, DEF_FILE and NSIS_DIR are all
  overridable from the command line so no files are written into the vendor
  tree at build time.
- Add tools/linux/overlay/include/: case-sensitivity shim headers
  (Windows.h, CommCtrl.h, MAPI.h, NTSecAPI.h, Psapi.h, ShlObj.h) that
  redirect to the lowercase MinGW system headers; also adds
  z7_idecl_noexcept_strip.h which resolves the noexcept-specifier mismatch
  in 7-zip 25.01 NSIS UI code.
- Move Nsis7z.def from versions/26.00/…/Bundles/Nsis7z/ to overlay/ (git
  rename; content unchanged).
- Delete versions/26.00/…/Bundles/Nsis7z/makefile.gcc (superseded by the
  overlay).
- Rewrite tools/linux/build_plugin_linux.py to drive the overlay makefile,
  support multiple 7-zip versions (25.01, 26.00, 26.01, zstd) with a
  VERSION_LAYOUT table, and add NSIS_DIR auto-detection for bundle-based
  builds.
- Add _linux_build/ to .gitignore.

tools/linux/overlay/include/CommCtrl.h

@@ -0,0 +1,3 @@
+/* Linux/MinGW cross-build shim: CommCtrl.h → commctrl.h */
+#pragma GCC system_header
+#include <commctrl.h>

tools/linux/overlay/include/MAPI.h

@@ -0,0 +1,3 @@
+/* Linux/MinGW cross-build shim: MAPI.h → mapi.h */
+#pragma GCC system_header
+#include <mapi.h>

tools/linux/overlay/include/NTSecAPI.h

@@ -0,0 +1,3 @@
+/* Linux/MinGW cross-build shim: NTSecAPI.h → ntsecapi.h */
+#pragma GCC system_header
+#include <ntsecapi.h>

tools/linux/overlay/include/Psapi.h

@@ -0,0 +1,3 @@
+/* Linux/MinGW cross-build shim: Psapi.h → psapi.h */
+#pragma GCC system_header
+#include <psapi.h>

tools/linux/overlay/include/ShlObj.h

@@ -0,0 +1,3 @@
+/* Linux/MinGW cross-build shim: ShlObj.h → shlobj.h */
+#pragma GCC system_header
+#include <shlobj.h>

tools/linux/overlay/include/Windows.h

@@ -0,0 +1,3 @@
+/* Linux/MinGW cross-build shim: Windows.h → windows.h */
+#pragma GCC system_header
+#include <windows.h>

tools/linux/overlay/include/z7_idecl_noexcept_strip.h

@@ -0,0 +1,33 @@
+#pragma GCC system_header
+/*
+ * Linux/MinGW cross-build shim for 7-zip 25.01 UI/NSIS sources.
+ *
+ * In 25.01, IDecl.h defines Z7_COM7F_E as throw().  Under GCC/C++17,
+ * throw() is treated as noexcept, causing a hard "different exception
+ * specifier" error when STDMETHODIMP out-of-class definitions (which carry
+ * no exception spec) are compiled against class declarations that used the
+ * noexcept-bearing Z7_COM7F_IMF macro.
+ *
+ * Strategy (no vendor files modified):
+ *   1. Include IDecl.h now so its include guard (ZIP7_INC_IDECL_H) is set.
+ *      Subsequent indirect includes of IDecl.h from the source file will be
+ *      no-ops.
+ *   2. Immediately undefine Z7_COM7F_E and redefine it as empty.
+ *      Because the preprocessor expands macros lazily (at use site, not at
+ *      definition site), all Z7_COM7F_IMF / Z7_COM7F_EO usages that follow
+ *      will pick up the empty definition, producing declarations with no
+ *      exception specifier — matching the STDMETHODIMP definitions.
+ */
+#include "7zip/IDecl.h"   /* searched via -I$(CPP_ROOT); resolves to versions/<ver>/CPP/7zip/IDecl.h */
+
+#undef  Z7_COM7F_E
+#define Z7_COM7F_E               /* empty – no exception specifier */
+
+/* Cascade: Z7_COM7F_EO and Z7_COM7F_EOF reference Z7_COM7F_E by name,
+ * so they pick it up automatically.  Redefine them explicitly to avoid
+ * "macro redefined" warnings if they were already expanded anywhere. */
+#undef  Z7_COM7F_EO
+#define Z7_COM7F_EO   Z7_COM7F_E Z7_override
+
+#undef  Z7_COM7F_EOF
+#define Z7_COM7F_EOF  Z7_COM7F_EO Z7_final