The dump_syms_mac tool only works for the system it is being built for
(it doesn't support running on ELFs for a diff target), and it builds
only for x86 currently.

If you look at the mac header:
	src/third_party/mac_headers/mach/machine/vm_types.h
it will #error for non x86/arm systems, and the arm header is not in
our source tree.

Tweak the build so it's only compiled when targetting x86 systems.

BUG=chromium:579384
TEST=`make check` pass
R=ted.mielczarek@gmail.com

Review URL: https://codereview.chromium.org/1645673002 .