[gdb/python] Fix abort on Py_Initialize

I tried out making python initialization fail by passing an incorrect
PYTHONHOME with python 3.6, and got:
...
$ PYTHONHOME=foo gdb -q
Fatal Python error: Py_Initialize: Unable to get the locale encoding
ModuleNotFoundError: No module named 'encodings'

Current thread 0x0000ffff89269c80 (most recent call first):

Fatal signal: Aborted
  ...
Aborted (core dumped)
$
...

This is as per spec: when Py_Initialize () fails, a fatal error is raised
using Py_FatalError.

This can be worked around using:
...
$ PYTHONHOME=foo gdb -q -eiex "set python ignore-environment on"
(gdb)
...
but it would be better if gdb didn't abort.

I found an article [1] describing two solutions:
- try out Py_Initialize in a separate process, and
- catch the abort using a signal handler.

This patch implements the latter solution.

Obviously we cannot call into python anymore after the abort, so we avoid
calling Py_IsInitialized (), and instead use a new variable py_isinitialized.

This gets us instead:
...
$ PYTHONHOME=foo gdb -q
Fatal Python error: Py_Initialize: Unable to get the locale encoding
ModuleNotFoundError: No module named 'encodings'

Current thread 0x0000fffecfd49c80 (most recent call first):
Python not initialized
$
...

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR python/32379
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32379

[1] https://stackoverflow.com/questions/7688374/how-to-i-catch-and-handle-a-fatal-error-when-py-initialize-fails

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