Upgrade musl builds to latest LLVM

[zanieb]

Right now, we're using LLVM for musl

python-build-standalone/pythonbuild/utils.py

Lines 480 to 482 in 01aa806

	# musl currently has issues with LLVM 15+.
	if "musl" in target_triple:
	return "llvm-14-x86_64-linux"

There are various problems that aren't worth investigating until we're on the newest compiler version. We should upgrade to LLVM 20.

[geofft]

Ugh, just ran into this with #827, let me take a stab at this and see if it's trivial with latest LLVM....

[zanieb]

It was definitely not trivial last time I tried.

fyi this is one of the tasks I sent to @jjhelmus — I don't mind if you do it but you have a bunch of other partially finished work that's higher priority too.

[geofft]

Yeah, it's looking not trivial, I only wanted to do it if fixing this properly was more trivial than working around the older LLVM versions in my other PR :)

It's failing on musl static builds only with "configure: error: cannot run C compiled programs." I think we unfortunately don't upload config.log anywhere on successful builds. See e.g.

https://github.com/astral-sh/python-build-standalone/actions/runs/18854882586/job/53800321366

(Several of the other runs in that PR failed from GitHub flakiness, the x86_64_* musl+static ones are the ones to look at.)

We do something slightly weird in cpython-unix/build-tcl.sh that seemed relevant to me:

       if echo "${TARGET_TRIPLE}" | grep -q -- "-unknown-linux-musl"; then
               # tcl misbehaves when performing static musl builds
               # `checking whether musl-clang accepts -g...` fails with a duplicate definition error
               TARGET_TRIPLE="$(echo "${TARGET_TRIPLE}" | sed -e 's/-unknown-linux-musl/-unknown-linux-gnu/g')"
       fi

I tried reverting it but it indeed fails with a duplicate definition error, see e.g.

https://github.com/astral-sh/python-build-standalone/actions/runs/18856614534/job/53806292643

The output is interspersed with ./configure output, I think ./configure is running successfully (which would be promising) and this is just wacky buffering but I'm not sure. Lightly edited, the relevant failure (I think) is

tcl> musl-clang -O2 -march=x86-64-v2 -fvisibility=hidden -fPIC -I/tools/deps/include -pipe -march=x86-64-v2 -fvisibility=hidden -fPIC -I/tools/deps/include  -march=x86-64-v2 -fvisibility=hidden -L/tools/deps/lib -Wl,--exclude-libs,ALL -Wl,--export-dynamic  tclAppInit.o \
tcl> 	-L/build/tcl8.6.14/unix -ltcl8.6 libtclstub8.6.a -ldl -lz  -lpthread   \
tcl> 	"-Wl,-rpath,/tools/deps/lib" -o tclsh
tcl> /tools/host/bin/ld: /tools/host/lib/libc.a(strtol.lo): in function `strtoul':
tcl> strtol.c:(.text.strtoul+0x0): multiple definition of `strtoul'; /build/tcl8.6.14/unix/libtcl8.6.a(strtoul.o):strtoul.c:(.text+0x0): first defined here
tcl> clang: error: linker command failed with exit code 1 (use -v to see invocation)
tcl> make: *** [tclsh] Error 1
tcl> Makefile:749: recipe for target 'tclsh' failed

And with that I'll leave it to @jjhelmus :-)