Bundle pkgconf with VitaSDK

[devnoname120]

Dependency

Depends on vitasdk/vita-toolchain#305.

This PR is intentionally a draft while #305 is unmerged. To exercise the coordinated change in CI, this branch temporarily points TOOLCHAIN_REPOSITORY at devnoname120/vita-toolchain and pins the exact signed #305 head, 9e3860c1617b8f55162851ff4093d3446093d8b9. Before this PR is merged, restore the official vitasdk/vita-toolchain URL and replace the pin with #305's eventual upstream squash commit.

Problem

VitaSDK currently installs the target-aware arm-vita-eabi-pkg-config frontend but does not ship its host pkg-config/pkgconf backend. A fresh installation therefore depends on the user installing a compatible host implementation separately. On Windows, that also leaves the native frontend introduced by #305 without a guaranteed sibling backend.

Changes

• build pinned pkgconf 3.0.7 as a host-native tool for every SDK host;
• install pkgconf/pkgconf.exe and its license in the core SDK;
• make finalization, SDK validation, core-package validation, and bootstrap validation require both the backend and Vita frontend;
• extend Unix and Windows bootstrap smoke tests to query a target .pc file from an SDK path containing spaces while hostile host PKG_CONFIG_* values are set;
• make the native Windows bootstrap smoke test configure an actual CMake project using find_package(PkgConfig) and pkg_check_modules(IMPORTED_TARGET ...) through #305's frontend;
• add package and SDK-fixture regression coverage for Unix and Windows layouts.

Verification

• Native Linux and MinGW pkgconf 3.0.7 ExternalProject builds pass.
• The Windows pkgconf.exe depends only on ADVAPI32.dll, KERNEL32.dll, and msvcrt.dll.
• Bootstrap/core package contract tests pass for Unix and Windows layouts.
• SDK validation fixtures reject missing frontend/backend pairs and accept complete pairs.
• The PowerShell bootstrap smoke-test contract passes.
• A coupled MinGW stage-2 superbuild built and installed both executables, and resolved a Vita target .pc file through Wine from an SDK path containing spaces.
• A separate end-to-end test using Windows CMake 3.31.6 under Wine successfully ran find_package(PkgConfig) and pkg_check_modules(IMPORTED_TARGET ...), selected package version 1.0, and ignored hostile host metadata.