squid: disable atomic operations for powerpc/mips

Don't enable GNU atomic operations for all targets, it fails on
powerpc and mips:

  AtomicWord.h: undefined reference to `__sync_fetch_and_add_8'
  collect2: error: ld returned 1 exit status

Refer to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56300

:

  There is no hardware support for 8 bytes atomic operations on
  32-bit MIPS targets.

The 32-bit PowerPC fails as well.

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>