gdb/testsuite: add max number of instructions check in gdb.base/unwind-on-each-insn.exp

gdb/testsuite: add max number of instructions check in gdb.base/unwind-on-each-insn.exp

This test sends my CI in an infinite loop of failures.   We expect to
have a handful of iterations (5 on my development machine, where the
test passes fine)but the log shows that it went up to 104340 iterations:

    FAIL: gdb.base/unwind-on-each-insn.exp - instruction 104340: maint print frame-id
    DUPLICATE: gdb.base/unwind-on-each-insn.exp - instruction 104340: maint print frame-id
    FAIL: gdb.base/unwind-on-each-insn.exp - instruction 104340: [string equal $fid $main_fid]
    FAIL: gdb.base/unwind-on-each-insn.exp - instruction 104340: get hexadecimal valueof "$pc"

Add a max instruction check, exit the loop if we reach 100 iterations.
This should allow the test to fail fast if there's a problem, but 100
iterations should be more than enough for when things are working.

Change-Id: I77978d593aca046068f9209272d82e1675ba17c2