linux-user/signal.c: Ensure AArch64 signal frame isn't too small

The AArch64 signal frame design was extended for SVE in commit
8c5931de, so that instead of having a fixed setup we
now add various records to the frame, with some of them possibly
overflowing into an extra space outside the original 4K reserved
block in the target_sigcontext.  However, we failed to ensure that we
always at least allocate the 4K reserved block.  This is ABI, and
some userspace programs rely on it.  In particular the dash shell
would segfault if the frame wasn't as big enough.

(Compare the kernel's sigframe_size() function in
arch/arm64/kernel/signal.c.)

Reported-by: Richard Henwood <richard.henwood@arm.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180409140714.26841-1-peter.maydell@linaro.org
Fixes: https://bugs.launchpad.net/bugs/1761535


Fixes: 8c5931de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>