Port to 2.1: Fix alternate stack cleanup on MUSL (#18687)

Port to 2.1: Fix alternate stack cleanup on MUSL (#18687)

The MUSL implementation of sigaltstack checks that the ss.ss_size is
larger or equal than the MINSIGSTKSZ even when the ss_flags is set
to SS_DISABLE even though Linux man page for sigaltstack states that
when this flag is set, all other ss fields are ignored.

We were not setting the ss_size in this case and it was causing a memory
leak for each thread that has terminated on MUSL based Linux distros
like Alpine.

Glibc implementation doesn't check the ss_size when the SS_DISABLE is set
so the problem was really MUSL specific.