build: fix host_arch_cc() for AIX/IBM i

The AIX/IBM i branch in `host_arch_cc()` that hardcodes the compiler
executable to `gcc` precludes picking up either `CC_host` or `CC`
environment variables (if set) as is done on other platforms. On an
AIX/IBM i platform where the compiler is, e.g. `gcc-10` instead of
just `gcc`, the current check will fail to detect the host architecture
and incorrectly default to `ia32`. Removing the AIX/IBM i specific
branch will follow the same logic as on the other platforms:
1. The value, if set, of the `CC_host` environment variable.
2. Otherwise, if set, the value of the `CC` environment variable.
3. `gcc` (`cc` if on macOS).

PR-URL: #39481
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Ash Cripps <acripps@redhat.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <midawson@redhat.com>

configure.py

@@ -964,12 +964,7 @@ def is_arm_hard_float_abi():
 def host_arch_cc():
   """Host architecture check using the CC command."""
 
-  if sys.platform.startswith('aix'):
-    # we only support gcc at this point and the default on AIX
-    # would be xlc so hard code gcc
-    k = cc_macros('gcc')
-  else:
-    k = cc_macros(os.environ.get('CC_host'))
+  k = cc_macros(os.environ.get('CC_host'))
 
   matchup = {
     '__aarch64__' : 'arm64',