Reduce test parallelism on ARM64 CI

Let's go from 32 to 16 parallel jobs. We can reduce this further
if necessary.

Closes phpGH-5426.

.travis.yml

@@ -89,7 +89,9 @@ before_script:
 
 # Run PHPs run-tests.php
 script:
- - ./sapi/cli/php run-tests.php -P -d extension=`pwd`/modules/zend_test.so $(if [ $ENABLE_DEBUG == 0 ]; then echo "-d opcache.enable_cli=1 -d opcache.protect_memory=1 -d zend_extension=`pwd`/modules/opcache.so"; fi) -g "FAIL,XFAIL,BORK,WARN,LEAK,SKIP" --offline --show-diff --show-slow 1000 --set-timeout 120 -j$(nproc)
+ # ARM64 CI reports nproc=32, which is excessive.
+ - if [ -z "$ARM64" ]; then export JOBS=$(nproc); else export JOBS=16; fi
+ - ./sapi/cli/php run-tests.php -P -d extension=`pwd`/modules/zend_test.so $(if [ $ENABLE_DEBUG == 0 ]; then echo "-d opcache.enable_cli=1 -d opcache.protect_memory=1 -d zend_extension=`pwd`/modules/opcache.so"; fi) -g "FAIL,XFAIL,BORK,WARN,LEAK,SKIP" --offline --show-diff --show-slow 1000 --set-timeout 120 -j$JOBS
  - sapi/cli/php -d extension_dir=`pwd`/modules -r 'dl("zend_test");'
 
 after_success: