Ignore `CMAKE_SYSROOT` when cross-compiling (#8318)

Ignore `CMAKE_SYSROOT` when cross-compiling (#8318)

This commit changes our build system to ignore CMAKE_SYSROOT for
find_file(), find_path(), and find_library() when cross-compiling.
Previously, it could happen that, e.g., when we were cross-compiling a
static build for Windows, CMake would pick up /usr/lib/libgmp.a
instead of cross-compiling GMP. The comments in the Toolchain-*.cmake
files suggest that the new behavior was the intended behavior all along.