meson: host_system value for Solaris is 'sunos' not 'solaris'.

meson: host_system value for Solaris is 'sunos' not 'solaris'.

This thinko caused us to not substitute our own getopt() code,
which results in failing to parse long options for the postmaster
since Solaris' getopt() doesn't do what we expect.  This can be seen
in the results of buildfarm member icarus, which is the only one
trying to build via meson on Solaris.

Per consultation with pgsql-release, it seems okay to fix this
now even though we're in release freeze.  The fix visibly won't
affect any other platforms, and it can't break Solaris/meson
builds any worse than they're already broken.

Discussion: https://postgr.es/m/2471229.1770499291@sss.pgh.pa.us
Backpatch-through: 16