Auto merge of #117465 - paulmenage:small-data-limit, r=compiler-errors

Add -Z small-data-threshold

This flag allows specifying the threshold size above which LLVM should not consider placing small objects in a `.sdata` or `.sbss` section.

Support is indicated in the target options via the
small-data-threshold-support target option, which can indicate either an
LLVM argument or an LLVM module flag.  To avoid duplicate specifications
in a large number of targets, the default value for support is
DefaultForArch, which is translated to a concrete value according to the
target's architecture.

Author: bors