Remove out-of-line atomic implementation for PowerPC

Remove out-of-line atomic implementation for PowerPC

Only support inline assembler with GCC. Support for xlC or other
compilers is not provided or tested. The files in
src/corelib/arch/vxworks only supported out-of-line PowerPC, so
remove it as well.

The xlC compiler seems to support GNU-style inline assembly, which we
prefer. Anyone wishing to support that compiler again should first test
if the inline assembly (the prefered method) works. See

http://publib.boulder.ibm.com/infocenter/lnxpcomp/v8v101/topic/com.ibm.xlcpp8l.doc/language/ref/asm.htm#asm

Change-Id: I70e56c0b06428bfb6538ca5e101baebd870f92f5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>