python*: set thread stack size on musl

Ensure recursion limit is reached before stack overflow.

Python does this for OSX and BSD:
13ff24582c/Python/thread_pthread.h (L22)

Size of 1MB chosen to match value in Alpine:
https://git.alpinelinux.org/cgit/aports/commit/main/python2/APKBUILD?id=2f35283fec8ec451fe5fb477dd32ffdcc0776e89

Manual testing via Alpine's test-stacksize.py crashes on these
previously, and works with these changes.

pkgs/development/interpreters/python/cpython/3.6/default.nix

@@ -122,6 +122,9 @@ in stdenv.mkDerivation {
     ${optionalString stdenv.isDarwin ''
        export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -msse2"
        export MACOSX_DEPLOYMENT_TARGET=10.6
+     ''
+     + optionalString stdenv.hostPlatform.isMusl ''
+      export NIX_CFLAGS_COMPILE+=" -DTHREAD_STACK_SIZE=0x100000"
      ''}
   '';