runtime: add "sigaction" to sigreturn symbol name

runtime: add "sigaction" to sigreturn symbol name

In order to identify the sigreturn function, gdb looks for
"__restore_rt". However because that symbol is sometimes missing from
the symbol table, it also performs the same instruction matching as
libgcc, but only in symbols containing "sigaction" (it expects sigaction
to preceed __restore_rt).

To match this heuristic, we add __sigaction to the sigreturn symbol
name.

Fixes #25218.

Change-Id: I09cb231ad23f668d451f31dd5633f782355fc91d
Reviewed-on: https://go-review.googlesource.com/c/go/+/479096
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>