gdb/testsuite: use `kill -FOO` instead of `kill -SIGFOO`

gdb/testsuite: use `kill -FOO` instead of `kill -SIGFOO`

When running gdb.base/bg-exec-sigint-bp-cond.exp when SHELL is dash,
rather than bash, I get:

    c&^M
    Continuing.^M
    (gdb) sh: 1: kill: Illegal option -S^M
    ^M
    Breakpoint 2, foo () at /home/jenkins/smarchi/binutils-gdb/build/gdb/testsuite/../../../gdb/testsuite/gdb.base/bg-exec-sigint-bp-cond.c:23^M
    23        return 0;^M
    FAIL: gdb.base/bg-exec-sigint-bp-cond.exp: no force memory write: SIGINT does not interrupt background execution (timeout)

This is because it uses the kill command built-in the dash shell, and
using the SIG prefix with kill does not work with dash's kill.  The
difference is listed in the documentation for bash's POSIX-correct mode
[1]:

    The kill builtin does not accept signal names with a ‘SIG’ prefix.

Replace SIGINT with INT in that test.

By grepping, I found two other instances (gdb.base/sigwinch-notty.exp
and gdb.threads/detach-step-over.exp).  Those were not problematic on my
system though.  Since they are done through remote_exec, they don't go
through the shell and therefore invoke /bin/kill.  On my Arch Linux,
it's:

    $ /bin/kill --version
    kill from util-linux 2.38.1 (with: sigqueue, pidfd)

and on my Ubuntu:

    $ /bin/kill --version
    kill from procps-ng 3.3.17

These two implementations accept "-SIGINT".  But according to the POSIX
spec [2], the kill utility should recognize the signal name without the
SIG prefix (if it recognizes them with the SIG prefix, it's an
extension):

    -s  signal_name
        Specify the signal to send, using one of the symbolic names defined
	in the <signal.h> header. Values of signal_name shall be recognized
	in a case-independent fashion, without the SIG prefix. In addition,
	the symbolic name 0 shall be recognized, representing the signal
	value zero. The corresponding signal shall be sent instead of SIGTERM.
    -signal_name
        [XSI] [Option Start]
        Equivalent to -s signal_name. [Option End]

So, just in case some /bin/kill implementation happens to not recognize
the SIG prefixes, change these two other calls to remove the SIG
prefix.

[1] https://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/kill.html

Change-Id: I81ccedd6c9428ab63b9261813f1905a18941f8da
Reviewed-By: Tom Tromey <tom@tromey.com>