[gdb/testsuite] Fix timeout in gdb.mi/user-selected-context-sync.exp with gcc-11

When running test-case gdb.mi/user-selected-context-sync.exp with gcc-11, we
get:
...
continue^M
Continuing.^M
FAIL: gdb.mi/user-selected-context-sync.exp: mode=all-stop: test_setup: \
  inferior 1: continue to breakpoint: continue thread 1.2 to infinite \
  loop breakpoint (timeout)
...

This is a regression since commit aa33ea68330 "testsuite, mi: avoid a clang
bug in 'user-selected-context-sync.exp'", which fixes a similar hang when
using clang.

The source before the commit contains:
...
  while (1);
...
and after the commit:
...
  int spin = 1;
  while (spin);
...

[ FWIW, I've filed a PR gcc/101011 - Inconsistent debug info for "while (1);"
to mention that gcc-11 has different behaviour for these two loops. ]

The problem is that:
- the test-case expects the behaviour that a breakpoint set
  on the while line will trigger on every iteration, and
- that is not guaranteed by either version of the loop.

Fix this by using a while loop with a dummy body:
...
  volatile int dummy = 0;
  while (1)
    dummy = !dummy;
...
and setting the breakpoint in the body.

Tested on x86_64-linux with clang 10.0.1, gcc-4.8, gcc 7.5.0 and gcc 11.1.1.

gdb/testsuite/ChangeLog:

2021-06-10  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/user-selected-context-sync.c (child_sub_function, main):
	Rewrite while (1) using dummy loop body.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2021-06-10  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.mi/user-selected-context-sync.c (child_sub_function, main):
+	Rewrite while (1) using dummy loop body.
+
 2021-06-10  Bhuvanendra Kumar N  <Bhuvanendra.KumarN@amd.com>
 
 	* gdb.base/foll-exec.c: convert the multi-line function call into

gdb/testsuite/gdb.mi/user-selected-context-sync.c

@@ -27,10 +27,10 @@ static pthread_barrier_t barrier;
 static void
 child_sub_function (void)
 {
-  /* Deliberately spin on a variable instead of plain 'while (1)' to
-     avoid the Clang bug https://bugs.llvm.org/show_bug.cgi?id=49614.  */
-  int spin = 1;
-  while (spin); /* thread loop line */
+  volatile int dummy = 0;
+  while (1)
+    /* Dummy loop body to allow setting breakpoint.  */
+    dummy = !dummy; /* thread loop line */
 }
 
 static void *
@@ -60,10 +60,10 @@ main (void)
 
   pthread_barrier_wait (&barrier);
 
-  /* Deliberately spin on a variable instead of plain 'while (1)' to
-     avoid the Clang bug https://bugs.llvm.org/show_bug.cgi?id=49614.  */
-  int spin = 1;
-  while (spin); /* main break line */
+  volatile int dummy = 0;
+  while (1)
+    /* Dummy loop body to allow setting breakpoint.  */
+    dummy = !dummy; /* main break line */
 
   return 0;
 }