Cross compile with LLVM and a sysroot for riscv64

[jjhelmus]

Use LLVM 22 and a Debian buster sysroot to cross-compile for riscv64.

[jjhelmus]

validating dist/cpython-3.14.4-riscv64-unknown-linux-gnu-lto-20260422T1228.tar.zst
Error: errors found
error: python/install/bin/python3.14 loads illegal library libgcc_s.so.1
error: python/install/lib/libpython3.14.so.1.0 loads illegal library libgcc_s.so.1
error: python/install/lib/python3.14/lib-dynload/_ctypes_test.cpython-314-riscv64-linux-gnu.so loads illegal library libgcc_s.so.1

libgcc_s.so.1 can be added to the allowed libraries, similar to armv7 targets. I am interested in why this occurs with LLVM and only when LTO is enabled.

[jjhelmus]

Locally python in including symbols that do come from libgcc_s:

# readelf -sW ./python_lto/install/bin/python | grep "GCC"
   596: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __extenddftf2@GCC_3.0 (9)
   597: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __trunctfdf2@GCC_3.0 (9)

These are coming from a few different places, Modules/mathmodule.o is one example. My guess is that without LTO these are inlined.