Add "linker=" option to target_compile to support testing

ChangeLog

@@ -1,8 +1,14 @@
 2020-06-22  Jacob Bachmeyer  <jcb62281+dev@gmail.com>
 
+	* doc/dejagnu.texi (target_compile procedure): Document new
+	"linker=" option to target_compile.
+
 	* lib/target.exp (default_target_compile): Actually use internal
 	option for separate linker provided for Go language support.  This
 	seems to have been accidentally omitted from Tom Tromey's patches.
+	(default_target_compile): Add "linker=" option, similar to
+	"compiler=" option, but the former overrides the latter when
+	building an executable.
 	(default_target_compile): Ensure that "--emit obj" is spaced from
 	previous item when used to compile Rust.  Use "--emit asm" instead
 	of "-S" when generating assembly from Rust sources.

doc/dejagnu.texi

@@ -4599,6 +4599,9 @@ this option is given multiple times, only the last use is significant.
 @item compiler=@var{command}
 Override the defaults and use @var{command} as the compiler.  If
 this option is given multiple times, only the last use is significant.
+@item linker=@var{command}
+Override the defaults and use @var{command} to build executables.  If
+this option is given multiple times, only the last use is significant.
 @item early_flags=@var{flags}
 Prepend @var{flags} to the set of arguments to be passed to the compiler.
 Multiple uses of this option specify additional arguments.
@@ -5898,4 +5901,4 @@ This makes @code{runtest} exit.  Abbreviation: @kbd{q}.
 @c  LocalWords:  subdirectory prepend prepended testsuite filename Expect's svn
 @c  LocalWords:  DejaGnu CVS RCS SCCS prepending subcommands Tcl Awk Readline
 @c  LocalWords:  POSIX KFAIL KPASS XFAIL XPASS hostname multitable gfortran
-@c  LocalWords:  rustc
+@c  LocalWords:  rustc executables

lib/target.exp

@@ -312,15 +312,15 @@ proc default_target_compile {source destfile type options} {
     }
 
     set early_flags ""
-    # linker_opts_order is one of "sources-then-flags", "flags-then-sources".
-    # The order matters for things like -Wl,--as-needed.  The default is to
-    # preserve existing behavior.
-    set linker_opts_order "sources-then-flags"
     set add_flags ""
     set libs ""
     set compiler_type "c"
     set compiler ""
     set linker ""
+    # linker_opts_order is one of "sources-then-flags", "flags-then-sources".
+    # The order matters for things like -Wl,--as-needed.  The default is to
+    # preserve existing behavior.
+    set linker_opts_order "sources-then-flags"
     set ldflags ""
     set dest [target_info name]
 
@@ -445,6 +445,10 @@ proc default_target_compile {source destfile type options} {
 	    regsub "^compiler=" $i "" tmp
 	    set compiler $tmp
 	}
+	if {[regexp "^linker=" $i]} {
+	    regsub "^linker=" $i "" tmp
+	    set linker $tmp
+	}
 	if {[regexp "^early_flags=" $i]} {
 	    regsub "^early_flags=" $i "" tmp
 	    append early_flags " $tmp"