modules : Make sure we include <map> in system.h.

It appears that many targets include the map header transitively in
other std headers included from system.h.  However there are some
editions of clang/libc++ in Xcode that do not, which results in a
bootstrap fail - since when resolver.h is included  there is then a
conflict in declaring abort().

The fix is to ensure that map is pulled in by system.h and before
resolver.h is included.  As a precautionary measure and to alert
anyone perhaps adding another header to resolver.h this patch also
gates the direct includes there on !IN_GCC.

c++tools/ChangeLog:

	* resolver.h: Do not include std headers directly when
	building in GCC.

gcc/cp/ChangeLog:

	* mapper-client.cc (INCLUDE_MAP): New; require map to be
	included from system.h.
	* mapper-resolver.cc (INCLUDE_MAP): Likewise.