[gdb/testsuite] Fix gdb.base/shlib-call.exp with gcc-8

[gdb/testsuite] Fix gdb.base/shlib-call.exp with gcc-8

When running test-case gdb.base/shlib-call.exp with gcc-8 instead of gcc-7, we
have:
...
 (gdb) step^M
-main () at /data/gdb_versions/devel/src/gdb/testsuite/gdb.base/shmain.c:42^M
-42        g = mainshr1(g);^M
-(gdb) PASS: gdb.base/shlib-call.exp: step out of shr2 epilogue to main
+main () at /data/gdb_versions/devel/src/gdb/testsuite/gdb.base/shmain.c:41^M
+41        g = shr2(g);^M
+(gdb) FAIL: gdb.base/shlib-call.exp: step out of shr2 epilogue to main
...

This is due to the fact that gcc-8 generates more precise line info, making
the instruction after the call to shr2 at 0x4008f1:
...
  4008e4:  8b 05 aa 07 20 00  mov    0x2007aa(%rip),%eax  # 601094 <g>
  4008ea:  89 c7              mov    %eax,%edi
  4008ec:  e8 1f fe ff ff     callq  400710 <shr2@plt>
  4008f1:  89 05 9d 07 20 00  mov    %eax,0x20079d(%rip)  # 601094 <g>
...
a "recommended breakpoint location":
...
  [0x00000287]  Special opcode 187: advance Address by 13 to 0x4008f1 and \
    Line by 0 to 41
...
so when stepping out of shr2, gdb steps back onto line 41, the line containing
the call to shr2.

Fix this by detecting this situation and adding an extra step to reach
line 42.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-06  Tom de Vries  <tdevries@suse.de>

	* gdb.base/shlib-call.exp: Add extra step to reach shmain.c:42, if
	necessary.