Changes

Modernize signal handling on Linux i386, PowerPC64, and s390x (#9543)

OCaml's signal handlers need to know the state of the processor when the signal was received. The approach standardized by the Single Unix specification is to use 3-argument signal handing functions and the SA_SIGINFO flag to sigaction. However, as the Linux man page for sigaction says,

Undocumented:
Before the introduction of SA_SIGINFO, it was also possible to get some
additional information, namely by using a sa_handler with a second
argument of type struct sigcontext. See the relevant Linux kernel
sources for details. This use is obsolete now.

For historical reasons, the i386, PowerPC and s390x Linux ports of OCaml still use the undocumented, obsolete approach, while the other Linux ports use the modern SA_SIGINFO approach. 

For consistency and future-proofing, this PR updates the i386, PowerPC64, and s390x Linux ports to use the modern approach to signal handling.  PowerPC32 was left as before because of technical subtleties and lack of hardware to test changes.

The new code for PowerPC 64 bits includes the trick proposed in PR#1795 to support Musl in addition to Glibc on ppc64le.
 (commit: 3a408ff)