[gdb/testsuite] Handle --target_board_info switch in save_vars

Add a --target_board_info switch to save_vars such that we can do:
...
save_vars { --target_board_info multilib_flags } {
    unset_board_info multilib_flags
    set_board_info multilib_flags "$override_multilib_flags"
}
...
and use it in gdb_compile_shlib.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-19  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (save_vars): Add --target_board_info support.
	(gdb_compile_shlib): Use save_vars.

---
 gdb/testsuite/lib/gdb.exp | 63 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 44 insertions(+), 19 deletions(-)

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index e91023093e..4aae1fffe6 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -2364,6 +2364,9 @@ proc proc_with_prefix {name arguments body} {
 # Run BODY in the context of the caller.  After BODY is run, the variables
 # listed in VARS will be reset to the values they had before BODY was run.
 #
+# A variable preceded by "--target_board_info" specifies a variable stored
+# in board_info for the target board, rather than a tcl variable.
+#
 # This is useful for providing a scope in which it is safe to temporarily
 # modify global variables, e.g.
 #
@@ -2386,9 +2389,28 @@ proc proc_with_prefix {name arguments body} {
 proc save_vars { vars body } {
     array set saved_scalars { }
     array set saved_arrays { }
+    array set saved_target_board_info { }
     set unset_vars { }
+    set unset_target_board_info { }
 
+    set target_board_info 0
     foreach var $vars {
+	if { $var == "--target_board_info" } {
+	    set target_board_info 1
+	    continue
+	}
+	if { $target_board_info } {
+	    set target_board_info 0
+	    global board board_info
+	    set board [target_info name]
+	    if { [info exists board_info($board,$var)] } {
+		set saved_target_board_info($var) [board_info $board $var]
+	    } else {
+		lappend unset_target_board_info $var
+	    }
+	    continue
+	}
+
 	# First evaluate VAR in the context of the caller in case the variable
 	# name may be a not-yet-interpolated string like env($foo)
 	set var [uplevel 1 list $var]
@@ -2415,10 +2437,23 @@ proc save_vars { vars body } {
 	uplevel 1 [list array set $var $value]
     }
 
+    foreach {var value} [array get saved_target_board_info] {
+	global board
+	set board [target_info name]
+	unset_board_info $var
+	set_board_info $var $value
+    }
+
     foreach var $unset_vars {
 	uplevel 1 [list unset -nocomplain $var]
     }
 
+    foreach var $unset_target_board_info {
+	global board
+	set board [target_info name]
+	unset_board_info $var
+    }
+
     if {$code == 1} {
 	global errorInfo errorCode
 	return -code $code -errorinfo $errorInfo -errorcode $errorCode $result
@@ -4422,32 +4457,22 @@ proc gdb_compile_shlib_1 {sources dest options} {
 proc gdb_compile_shlib {sources dest options} {
     global board
 
-    # Save multilib_flags.
-    set board [target_info name]
-    set save_multilib_flag [board_info $board multilib_flags]
-
     # Ignore PIE-related setting in multilib_flags.
-    set multilib_flag ""
-    foreach op $save_multilib_flag {
+    set board [target_info name]
+    set multilib_flags_orig [board_info $board multilib_flags]
+    set multilib_flags ""
+    foreach op $multilib_flags_orig {
 	if { $op == "-pie" || $op == "-no-pie" \
 		 || $op == "-fPIE" || $op == "-fno-PIE"} {
 	} else {
-	    append multilib_flag " $op"
+	    append multilib_flags " $op"
 	}
     }
-    unset_board_info "multilib_flags"
-    set_board_info multilib_flags "$multilib_flag"
-    set code [catch {gdb_compile_shlib_1 $sources $dest $options} result]
-
-    # Restore multilib_flags.
-    unset_board_info "multilib_flags"
-    set_board_info multilib_flags $save_multilib_flag
 
-    if {$code == 1} {
-	global errorInfo errorCode
-	return -code error -errorinfo $errorInfo -errorcode $errorCode $result
-    } elseif {$code > 1} {
-	return -code $code $result
+    save_vars { --target_board_info multilib_flags } {
+	unset_board_info multilib_flags
+	set_board_info multilib_flags "$multilib_flags"
+	set result [gdb_compile_shlib_1 $sources $dest $options]
     }
 
     return $result