[vcpkg] CMAKE_SYSTEM_PROCESSOR is missing if VCPKG_TARGET_ARCHITECTUR…

…E is arm or arm64.

This change tries to fix issue microsoft#13395.

Root cause:
In script mode, cmake won't populate CMAKE_SYSTEM_PROCESSOR parameter automatically. That parameter is
required by libpng to configure build parameters. To fix this issue, we need explicitly set CMAKE_SYSTEM_PROCESSOR
value.

Verify:
On arm64-linux host, run `./vcpkg install tesseract:arm64-linux`.

scripts/toolchains/linux.cmake

@@ -10,6 +10,10 @@ elseif(VCPKG_TARGET_ARCHITECTURE STREQUAL "x86")
    set(CMAKE_SYSTEM_PROCESSOR x86 CACHE STRING "")
    string(APPEND VCPKG_C_FLAGS " -m32")
    string(APPEND VCPKG_CXX_FLAGS " -m32")
+elseif(VCPKG_TARGET_ARCHITECTURE STREQUAL "arm")
+   set(CMAKE_SYSTEM_PROCESSOR armv7l CACHE STRING "")
+elseif(VCPKG_TARGET_ARCHITECTURE STREQUAL "arm64")
+   set(CMAKE_SYSTEM_PROCESSOR aarch64 CACHE STRING "")
 endif()
 
 get_property( _CMAKE_IN_TRY_COMPILE GLOBAL PROPERTY IN_TRY_COMPILE )