Define macros for renaming xmalloc and xcalloc

Define macros for renaming xmalloc and xcalloc

I'm planning to link strace with libiberty for mangling C++
symbol names in stack trace enabled with -k option.

Both names, xmalloc and xcalloc, are already defined and used
in libiberty, and they conflict with functions in strace.
Rename xmalloc and xcalloc functions defined in strace
to strace_xmalloc and strace_xcalloc, respectively,
to avoid this conflict.

* xmalloc.h (xcalloc, xmalloc): New macros.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>