gdb: Use SYM_DOMAIN instead of DOMAIN when calling sym-domains.def

gdb: Use SYM_DOMAIN instead of DOMAIN when calling sym-domains.def

Since commit 6771fc6f1d9 "Use a .def file for domain_enum", the
sym-domains.def file has been introduced, and requires the user to
define the DOMAIN(x) macro.

On older systems (centos-7 with glibc-2.17 for example), this DOMAIN
macro conflicts with another macro defined in /usr/include/math.h.

Fix this conflict by changing sym-domains.def to use a macro named
SYM_DOMAIN instead of DOMAIN.

Change-Id: I679df30e2bd2f4333343f16bbd2a3511a37550a3
Approved-By: Tom Tromey <tom@tromey.com>