gallivm: fix FTBFS on i386 with LLVM >= 13, StackAlignmentOverride is gone

Upstream change: https://github.com/llvm/llvm-project/commit/3787ee457173c3612aac4c9b1a2b6d6ab0202616

We're now setting the alignment through Module::setOverrideStackAlignment by way of a new wrapper, since Module::getOverrideStackAlignment is not available in the C API of LLVM.

• Signed-off-by: Kai Wasserbäch kai@dev.carbon-project.org
• Closes: #4906 (closed)