[m-rev.] for review: Fix access to PC from signal handler context.

[Julien Fischer]

On Tue, 13 Oct 2020, Peter Wang wrote:

> configure.ac:
>    Deduplicate checks for how to access the program counter from a
>    signal handler context.
>
>    Check REG_RIP for x86-64 and REG_EIP for i386.
>
>    Define _GNU_SOURCE for the register constants (REG_RIP, etc.)
>
>    Drop support for accessing the PC on Alpha as it doesn't fit easily
>    into the "uc_mcontext.gregs[$reg]" pattern.

That's fine.

Julien.