gh-149021: Fix AIX build by dropping -e from SHELL in Makefile

[pratyush618]

On AIX, make(1) treats the value of SHELL as a literal executable
path rather than a command line. Since gh-100220 the top-level Makefile
sets SHELL = /bin/sh -e to make complex recipes fail-fast under GNU
make, but AIX then tries to execve("/bin/sh -e", ...) and the build
aborts immediately:

$ make
/bin/sh -e: not found
make: 1254-004 The error code from the last command is 1.

POSIX requires make to invoke the shell with -e by default; GNU
make does the same once a .POSIX: special target is declared (it sets
.SHELLFLAGS = -ec). This PR:

• adds .POSIX: as the first non-comment line in Makefile.pre.in;
• restores SHELL = /bin/sh so the value is a valid path on AIX;
• keeps the original fail-fast guarantee from make sharedinstall does not return failure if install commands fail #100220 on both POSIX and
  GNU make.

Verified locally with GNU Make 4.3 that a multi-command recipe
(echo a; false; echo b) aborts after false with .POSIX: set,
matching the prior SHELL = /bin/sh -e behavior.

• Issue: Invalid SHELL in Makefile for AIX builds? #149021

[picnixz]

1. Not sure it's the best way to fix it, we shouldn't change the build for all platforms. It could done for AIX-only in some sense.
2. We have AIX buildbots I think so it's surprising that this wasn't caught. I want to understand the failure more precisely.
3. Don't use LLMs to generate PRs. Read https://devguide.python.org/getting-started/generative-ai/ first please.
4. I'm not entirely sure we assume GNU Make. Maybe we should set .SHELLOPTIONS ourselves? or do some other tricks?

For those reasons, I'm closing the PR until we have a cleaner fix that doesn't mess with the entire build system.