Rollup merge of #113525 - workingjubilee:handle-dynamic-minsigstksz, r=m-ou-se

Dynamically size sigaltstk in std

On modern Linux with Intel AMX and 1KiB matrices,
Arm SVE with potentially 2KiB vectors,
and RISCV Vectors with up to 16KiB vectors,
we must handle dynamic signal stack sizes.

We can do so unconditionally by using getauxval,
but assuming it may return 0 as an answer,
thus falling back to the old constant if needed.

Fixes rust-lang/rust#107795

Author: matthiaskrgr