sim: filter out SIGSTKSZ [PR sim/28302]

sim: filter out SIGSTKSZ [PR sim/28302]

We map target signals to host signals so we can propagate signals
between the host & simulated worlds.  That means we need to know
the symbolic names & values of all signals that might be sent.

The tools that generate that list use signal.h and include all
symbols that start with "SIG" so as to automatically include any
new symbols that the C library might add.  Unfortunately, this
also picks up "SIGSTKSZ" which is not actually a signal itself,
but a signal related setting -- it's the size of the stack when
a signal is handled.

By itself this doesn't super matter as we will never see a signal
with that same value (since the range of valid signals tend to be
way less than 1024, and the size of the default signal stack will
never be that small).  But with recent glibc changes that make this
into a dynamic value instead of a compile-time constant, some users
see build failures when building the sim.

As suggested by Adam Sampson, update our scripts to ignore this
symbol to simplify everything and avoid the build failure.

Bug: https://sourceware.org/PR28302