This updates the list of builtins to the one in LLVM 17 (which is our
current version) and addes GENERIC_TF_SOURCES, which contains long
double floating point builtins (80 bit, 128 bit, etc).

This is needed for full printf support in the next commit.

This flushes stdio, so that functions like puts and printf write out all
buffered data even if the output isn't connected to a terminal.

For discussion, see:
bytecodealliance/wasmtime#7833

The C printf function is sometimes needed for C files included using
CGo. This commit makes sure they're available on all systems where CGo
is fully supported (that is, everywhere except on AVR).

For baremetal systems using picolibc, I've picked the integer-only
version of printf to save on flash size. We might want to consider
providing a way to pick the floating point version instead, if needed.