Disable zlib tests on x86_64-apple-darwin

[fmarier]

These are broken when cross-compiling for x86_64 on arm64:

12:03:26  gcc -dynamiclib -install_name /Users/jenkins/jenkins/workspace/brave-tor-client-build-macos/x86_64/zlib-1.3.1/root/lib/libz.1.dylib -compatibility_version 1 -current_version 1.3.1 -target x86_64-apple-macos11 -fPIC -DHAVE_HIDDEN -o libz.1.3.1.dylib adler32.lo crc32.lo deflate.lo infback.lo inffast.lo inflate.lo inftrees.lo trees.lo zutil.lo compress.lo uncompr.lo gzclose.lo gzlib.lo gzread.lo gzwrite.lo  -lc -target x86_64-apple-macos11

2:03:26 
 /bin/sh: ./minigzip: Bad CPU type in executable
12:03:26 
 /bin/sh: ./minigzip: Bad CPU type in executable
12:03:26  		*** zlib test FAILED ***
12:03:26 
 make: *** [teststatic] Error 1
12:03:26  make: *** Waiting for unfinished jobs....
12:03:26  /bin/sh: ./minigzipsh: Bad CPU type in executable
12:03:26  /bin/sh: ./minigzipsh: Bad CPU type in executable
12:03:26  		*** zlib shared test FAILED ***
12:03:26  make: *** [testshared] Error 1

[fmarier]

The x86_64 build is still broken, but for other reasons. The zlib tests are correctly ignored and so I'll merge.