runtime: shrink signal stacks with AT_MINSIGSTKSZ

[prattmic]

A new auxiliary vector, AT_MINSIGSTKSZ was added for arm64 in Linux 4.18, amd64 in Linux 5.14, and ppc64 in Linux 5.19.

AT_MINSIGSTKSZ reports the size of a signal frame, and thus the absolute minimum amount of space required to receive a signal, though additional space is of course required for userspace stack usage.

Currently we hard-code our signal stacks to 32KB. With AT_MINSIGSTKSZ, I think we could adjust this to something like AT_MINSIGSTKSZ + _StackFixed, which would almost certainly reduce the signal stack size on all systems (AT_MINSIGSTKSZ is 3632 bytes on my Skylake machine).

cc @aclements @cherrymui

[cherrymui]

Sounds like a good idea. Thanks.

Currently there are code that depend on the signal stack being its own manual span. That needs to be adjusted if the stack is smaller and allocated from a stack pool.