runtime: add special handling for signal 34

The musl libc uses signal 34 internally for setgid (similar to how glibc
uses signal 32 and signal 33).  For this reason, special handling is
needed for this signal in the runtime. The gc implementation already
handles the signal accordingly.  As such, this commit intends to
simply copy the behavior of the Google Go implementation to libgo.

See https://go.dev/issues/39343

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/400594