testsuite, fortran: Fix regression due to fix for ifort's 'start' behavior

testsuite, fortran: Fix regression due to fix for ifort's 'start' behavior

Got a regression email due to merge of commit in CI config
tcwg_gdb_check/master-aarch64 :
https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=41439185cd0075bbb1aedf9665685dba0827cfec

Begining of test "gdb.fortran/array-slices-bad.exp" was updated in above
commit to start the test from running to line with tag "First Breakpoint"
instead of "fortran_runto_main".  Reason of the regression is shared
libraries are still loaded after hitting the breakpoint as "nosharedlibrary"
is already called before hitting the breakpoint.

So now after this change test is updated accordingly to disable and unload
shared libraries symbols after hitting the first breakpoint.

Approved-By: Andrew Burgess <aburgess@redhat.com>