gdb/testsuite: Update gdb.base/so-impl-ld.exp

gdb/testsuite: Update gdb.base/so-impl-ld.exp

gdb.base/so-impl-ld.exp was setup assuming that the compiler would add
epilogue information and that GDB would stop in the } line.  This would
make clang tests fail like so:

 step^M
 solib_main (arg=10000) at ../../../common/git-repos/binutils-gdb/gdb/testsuite/gdb.base/solib1.c:7^M
 7|__  return arg*arg;|__|___/* HERE */^M
 (gdb) PASS: gdb.base/so-impl-ld.exp: step into solib call
 next^M
 main () at ../../../common/git-repos/binutils-gdb/gdb/testsuite/gdb.base/so-impl-ld.c:22^M
 22|_  return 0;^M
 (gdb) FAIL: gdb.base/so-impl-ld.exp: step in solib call
 next^M
 0x00007ffff7cef560 in __libc_start_call_main () from /lib64/libc.so.6^M
 (gdb) FAIL: gdb.base/so-impl-ld.exp: step out of solib call

This patch changes it so solib_main has 2 lines where GDB can stop
regardless of compiler choices, and updates the exp file to
generically deal with unknown number of steps until leaving that
function.