gdb/testsuite: Reduce gdb.threads/threadcrash.exp reliance on libc symbols

The test gdb.threads/threadcrash.exp demanded GDB to fully unwind and
print the names of all functions. However, some of the functions are
from the libc library, and so the test implicitly demanded libc symbols
to be available, and would fail otherwise, as was raised in PR
gdb/31293.

This commit changes it so we only explicitly check for functions that
are not provided by threadcrash.c if they are indeed available.

Tested on arm-linux and x86_64-linux.

Approved-By: Tom de Vries <tdevries@suse.de>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31293

gdb/testsuite/gdb.threads/threadcrash.exp

@@ -20,6 +20,9 @@
 # a gcore.
 
 
+set have_sleep -1
+set have_pthread_kill -1
+
 # Use 'thread apply all backtrace' to check if all expected threads
 # are present, and stopped in the expected locations.  Set the global
 # TEST_LIST to be the a list of regexps expected to match all the
@@ -27,6 +30,8 @@
 # GDB sees the threads.
 
 proc thread_apply_all {} {
+    global have_sleep
+    global have_pthread_kill
     global test_list
 
     set test_list { }
@@ -73,6 +78,14 @@ proc thread_apply_all {} {
 	    lappend test_list 7
 	    exp_continue
 	}
+	-re "^\r\n${hs} in sleep $hs$eol" {
+	    set have_sleep 1
+	    exp_continue
+	}
+	-re "^\r\n${hs} in pthread_kill $hs$eol" {
+	    set have_pthread_kill 1
+	    exp_continue
+	}
 	-re "^\r\n$hs$eol" {
 	    exp_continue
 	}
@@ -82,6 +95,13 @@ proc thread_apply_all {} {
     }
 
     gdb_assert {$unwind_fail == false}
+
+    if { $have_sleep == -1 } {
+	set have_sleep 0
+    }
+    if { $have_pthread_kill == -1 } {
+	set have_pthread_kill 0
+    }
 }
 
 # Perform all the tests we're interested in.  They are:
@@ -90,6 +110,8 @@ proc thread_apply_all {} {
 # * testing if GDB recreated the full backtrace we expect for all threads
 
 proc do_full_test {} {
+    global have_sleep
+    global have_pthread_kill
     global test_list
     set thread_count [get_valueof "" "\$_inferior_thread_count" 0]
     gdb_assert {$thread_count == 7}
@@ -98,6 +120,18 @@ proc do_full_test {} {
 
     gdb_assert {$thread_count == [llength $test_list]}
 
+    if { $have_sleep } {
+	set sleep ".*sleep.*"
+    } else {
+	set sleep ".*"
+    }
+
+    if { $have_pthread_kill } {
+	set pthread_kill ".*pthread_kill.*"
+    } else {
+	set pthread_kill ".*"
+    }
+
     for {set i 0} {$i < $thread_count } {incr i} {
 	set thread_num [expr [llength $test_list] - $i]
 
@@ -105,25 +139,25 @@ proc do_full_test {} {
 	if { $type == 1 } {
 	    set re \
 		[multi_line \
-		     ".sleep.*" \
+		     $sleep \
 		     ".*do_syscall_task .location=SIGNAL_ALT_STACK.*" \
 		     ".*signal_handler.*" \
 		     ".*signal handler called.*" \
-		     ".*pthread_kill.*" \
+		     $pthread_kill \
 		     ".*thread_function.*"]
 	} elseif { $type == 2 } {
 	    set re \
 		[multi_line \
-		     ".*sleep.*" \
+		     $sleep \
 		     ".*do_syscall_task .location=SIGNAL_HANDLER.*" \
 		     ".*signal_handler.*" \
 		     ".*signal handler called.*" \
-		     ".*pthread_kill.*" \
+		     $pthread_kill \
 		     ".*thread_function.*"]
 	} elseif { $type == 3 } {
 	    set re \
 		[multi_line \
-		     ".*sleep.*" \
+		     $sleep \
 		     ".*do_syscall_task .location=NORMAL.*" \
 		     ".*thread_function.*"]
 	} elseif { $type == 4 } {
@@ -132,7 +166,7 @@ proc do_full_test {} {
 		     ".*do_spin_task .location=SIGNAL_ALT_STACK.*" \
 		     ".*signal_handler.*" \
 		     ".*signal handler called.*" \
-		     ".*pthread_kill.*" \
+		     $pthread_kill \
 		     ".*thread_function.*"]
 	} elseif { $type == 5 } {
 	    set re \
@@ -140,7 +174,7 @@ proc do_full_test {} {
 		     ".*do_spin_task .location=SIGNAL_HANDLER.*" \
 		     ".*signal_handler.*" \
 		     ".*signal handler called.*" \
-		     ".*pthread_kill.*" \
+		     $pthread_kill \
 		     ".*thread_function.*"]
 	} elseif { $type == 6 } {
 	    set re \