[gdb/testsuite] Fix step-reverse.c with gcc-10

[gdb/testsuite] Fix step-reverse.c with gcc-10

The file gdb.reverse/step-reverse.c is used in test-cases:
- gdb.reverse/step-reverse.exp
- gdb.reverse/next-reverse-bkpt-over-sr.exp
- gdb.reverse/step-precsave.exp

With gcc-7, there are only PASSes (apart from one KFAIL), but with gcc-10, we
have the following FAILs:
...
FAIL: gdb.reverse/step-reverse.exp: reverse stepi from a function call \
  (start statement)
FAIL: gdb.reverse/step-reverse.exp: simple reverse stepi
FAIL: gdb.reverse/step-reverse.exp: reverse step out of called fn
FAIL: gdb.reverse/step-reverse.exp: reverse next over call
FAIL: gdb.reverse/step-reverse.exp: reverse step test 1
FAIL: gdb.reverse/step-reverse.exp: reverse next test 1
FAIL: gdb.reverse/step-reverse.exp: reverse step test 2
FAIL: gdb.reverse/step-reverse.exp: reverse next test 2
FAIL: gdb.reverse/step-precsave.exp: reverse stepi from a function call \
  (start statement)
FAIL: gdb.reverse/step-precsave.exp: simple reverse stepi
FAIL: gdb.reverse/step-precsave.exp: reverse step out of called fn
FAIL: gdb.reverse/step-precsave.exp: reverse next over call
FAIL: gdb.reverse/step-precsave.exp: reverse step test 1
FAIL: gdb.reverse/step-precsave.exp: reverse next test 1
FAIL: gdb.reverse/step-precsave.exp: reverse step test 2
FAIL: gdb.reverse/step-precsave.exp: reverse next test 2
...

Looking at the first step-precsave.exp FAIL, we have:
...
(gdb) stepi^M
26        myglob++; return 0;   /* ARRIVED IN CALLEE */^M
(gdb) PASS: gdb.reverse/step-precsave.exp: reverse stepi thru function return
stepi^M
0x000000000040055f      26        myglob++; return 0;   /* ARRIVED IN CALLEE */^M
(gdb) FAIL: gdb.reverse/step-precsave.exp: reverse stepi from a function call \
  (start statement)
...

There's a difference in line info for callee:
...
    25  int callee() {          /* ENTER CALLEE */
    26    myglob++; return 0;   /* ARRIVED IN CALLEE */
    27  }                       /* RETURN FROM CALLEE */
...
between gcc-7:
...
Line number    Starting address    View    Stmt
         25            0x400557               x
         26            0x40055b               x
         27            0x40056f               x
...
and gcc-10:
...
         25            0x400552               x
         26            0x400556               x
         26            0x400565               x
         27            0x40056a               x
...

The two "recommend breakpoint location" entries at line 26 are for the two
statements ("myglob++" and "return 0"), but the test-case expects to hit line
26 only once.

Fix this by rewriting the two statements into a single statement:
...
-  myglob++; return 0;  /* ARRIVED IN CALLEE */
+  return myglob++;     /* ARRIVED IN CALLEE */
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-21  Tom de Vries  <tdevries@suse.de>

	* gdb.reverse/step-reverse.c (callee): Merge statements.