gdb: target_waitstatus_to_string: print extra info for FORKED, VFORKED, EXECD

gdb: target_waitstatus_to_string: print extra info for FORKED, VFORKED, EXECD

Print the extra information contained in target_waitstatus for these
events.  For TARGET_WAITKIND_{FORKED,VFORKED}, the extra information is
contained in related_pid, and is the ptid of the new process.  For
TARGET_WAITKIND_EXECD, it,s the exec'd path name in execd_pathname.
Print it using the same format used for TARGET_WAITKIND_STOPPED and
others.

Here are sample outputs for all three events:

    [infrun] print_target_wait_results: target_wait (-1.0.0 [process -1], status) =
    [infrun] print_target_wait_results:   726890.726890.0 [process 726890],
    [infrun] print_target_wait_results:   status->kind = vforked, related_pid = 726894.726894.0

    [infrun] print_target_wait_results: target_wait (-1.0.0 [process -1], status) =
    [infrun] print_target_wait_results:   727045.727045.0 [process 727045],
    [infrun] print_target_wait_results:   status->kind = forked, related_pid = 727049.727049.0

    [infrun] print_target_wait_results: target_wait (-1.0.0 [process -1], status) =
    [infrun] print_target_wait_results:   727119.727119.0 [process 727119],
    [infrun] print_target_wait_results:   status->kind = execd, execd_pathname = /usr/bin/ls

Change-Id: I4416a74e3bf792a625a68bf26c51689e170f2184