Fix ./configure checks with __cpuidex() and __cpuid()

Fix ./configure checks with __cpuidex() and __cpuid()

The configure checks used two incorrect functions when checking the
presence of some routines in an environment:
- __get_cpuidex() for the check of __cpuidex().
- __get_cpuid() for the check of __cpuid().
This means that Postgres has never been able to detect the presence of
these functions, impacting environments where these exist, like Windows.

Simply fixing the function name does not work.  For example, using
configure with MinGW on Windows causes the checks to detect all four of
__get_cpuid(), __get_cpuid_count(), __cpuidex() and __cpuid() to be
available, causing a compilation failure as this messes up with the
MinGW headers as we would include both <intrin.h> and <cpuid.h>.

The Postgres code expects only one in { __get_cpuid() , __cpuid() } and
one in { __get_cpuid_count() , __cpuidex() } to exist.  This commit
reshapes the configure checks to do exactly what meson is doing, which
has been working well for us: check one, then the other, but never allow
both to be detected in a given build.

The logic is wrong since 3dc2d62d0486 and 792752af4eb5 where these
checks have been introduced (the second case is most likely a copy-pasto
coming from the first case), with meson documenting that the configure
checks were broken.  As far as I can see, they are not once applied
consistently with what the code expects, but let's see if the buildfarm
has different something to say.  The comment in meson.build is adjusted
as well, to reflect the new reality.

Author: Lukas Fittl <lukas@fittl.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aIgwNYGVt5aRAqTJ@paquier.xyz
Backpatch-through: 13