linux-user: Disable static assert involving __SIGRTMAX if it is missing

linux-user: Disable static assert involving __SIGRTMAX if it is missing

This check is to ensure that the loop in signal_table_init() from
SIGRTMIN to SIGRTMAX falls within the bounds of host_to_target_signal_table
(_NSIG). However, it is not critical, since _NSIG is already defined
to be the one larger than the largest signal supported by the system
(as specified in the upcoming POSIX revision[0]).

musl libc does not define __SIGRTMAX, so disabling this check when
it is missing fixes one of the last remaining errors when building
qemu.

[0] https://www.austingroupbugs.net/view.php?id=741

Signed-off-by: Michael Forney <mforney@mforney.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210526190203.4255-1-mforney@mforney.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>