and add a fallback for unsupported operating systems in detect()

In 589c74a the cpuid detection was changed to use systemcfg, but a copy
and paste error was introduced during some refactoring that caused
POWER7 detection to reference CPUTYPE_POWER7 (which doesn't exist)
instead of CPUTYPE_POWER6.

AIX libc already provides ready access to an integer that contains a bit
identifying the CPU it's running on, so there's no need to call a
program and grep its output. Additionally, prtconf is not available in
the PASE runtime, which provides an AIX emulation layer on the IBM i
operating system.

The AIX systemcfg.h also provides macro definitions like POWER_8,
POWER_9, etc for all the bits defining the CPUs as well as macros like
__power_8(), __power_9_andup() that return booleans, but I did not use
them. Since these macros depend on the level of the OS in which it is
built, they may not be defined and instead the associated hex literals
are used directly.

This is the initial patch to support build infrastructure
for POWER10 architecture.

__asm is complete on its own

DBSD developers state they will only support amd64, but NetBSD supports POWER.

(already supported by the *BSD version)

for #1803

fixes #1671

Signed-off-by: Timothy Gu <timothygu99@gmail.com>