[clang] Remove old Linux kernel workaround for ensuring stack space (#81533)

PR #71709 broke the Linux PIE build with `undefined symbol: alloca`
errors. With the newly included `clang/Basic/Builtins.h` in that PR, it
surfaces an issue with a combination of two previous patches.

26670dcb added `#undef alloca` so clang
builtins handling of alloca would work under MSVC (unsure if this is
still necessary).

194b6a3b added code that calls `alloca`
to workaround a Linux kernel < 4.1 bug. Given that Linux 4.1 was EOL in
2018, it should be ok to remove this workaround.