d: Fix documentation of -defaultlib= and -debuglib=

From the generated manpages, it was not clear that its usage is
'-debuglib=<libname>'.

gcc/d/ChangeLog:

	* gdc.texi (Options for Linking): Clarify usage of -defaultlib= and
	-debuglib= options.

gcc/d/ChangeLog

+2020-04-30  Iain Buclaw  <ibuclaw@gdcproject.org>
+
+	* gdc.texi (Options for Linking): Clarify usage of -defaultlib= and
+	-debuglib= options.
+
 2020-04-29  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	* d-codegen.cc (d_decl_context): Don't include module in the name of

gcc/d/gdc.texi

@@ -665,14 +665,14 @@ a link step.
 
 @table @gcctabopt
 
-@item -defaultlib @var{libname}
-@cindex @option{-defaultlib}
+@item -defaultlib=@var{libname}
+@cindex @option{-defaultlib=}
 Specify the library to use instead of libphobos when linking.  Options
 specifying the linkage of libphobos, such as @option{-static-libphobos}
 or @option{-shared-libphobos}, are ignored.
 
-@item -debuglib
-@cindex @option{-debuglib}
+@item -debuglib=@var{libname}
+@cindex @option{-debuglib=}
 Specify the debug library to use instead of libphobos when linking.
 This option has no effect unless the @option{-g} option was also given
 on the command line.  Options specifying the linkage of libphobos, such