Don't override operator new if GDB is built with -fsanitize=address

Don't override operator new if GDB is built with -fsanitize=address

Nowadays, if we build GDB with -fsanitize=address, we can get the asan
error below,

(gdb) quit
=================================================================
==9723==ERROR: AddressSanitizer: alloc-dealloc-mismatch (malloc vs operator delete) on 0x60200003bf70
    #0 0x7f88f3837527 in operator delete(void*) (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x55527)
    #1 0xac8e13 in __gnu_cxx::new_allocator<void (*)()>::deallocate(void (**)(), unsigned long) /usr/include/c++/4.9/ext/new_allocator.h:110
    #2 0xac8cc2 in __gnu_cxx::__alloc_traits<std::allocator<void (*)()> >::deallocate(std::allocator<void (*)()>&, void (**)(), unsigned long) /usr/include/c++/4.9/ext/alloc_traits.h:185
....
0x60200003bf70 is located 0 bytes inside of 8-byte region [0x60200003bf70,0x60200003bf78)
allocated by thread T0 here:
    #0 0x7f88f38367ef in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x547ef)
    #1 0xbd2762 in operator new(unsigned long) /home/yao/SourceCode/gnu/gdb/git/gdb/common/new-op.c:42
    #2 0xac8edc in __gnu_cxx::new_allocator<void (*)()>::allocate(unsigned long, void const*) /usr/include/c++/4.9/ext/new_allocator.h:104
    #3 0xac8d81 in __gnu_cxx::__alloc_traits<std::allocator<void (*)()> >::allocate(std::allocator<void (*)()>&, unsigned long) /usr/include/c++/4.9/ext/alloc_traits.h:182

The reason for this is that we override operator new but don't override
operator delete.  This patch does the override if the code is NOT
compiled with asan.

gdb:

2016-10-25  Yao Qi  <yao.qi@linaro.org>

	PR gdb/20716
	* common/new-op.c (__has_feature): New macro.
	Don't override operator new if asan is used.