Obsolete Solaris 10 support

Obsolete Solaris 10 support

Here's the patch corresponding to the Solaris 10 obsoletion announcement

	https://sourceware.org/ml/gdb/2019-10/msg00008.html

Right now it doesn't remove any code, but obviates the need to test on
that ancient platform.  Besides, some of the patches I have in my queue
would require different solutions for Solaris 10 and 11.

There are a few comment-only references that I've kept since they are
still correct as is, even when GDB doesn't support Solaris 10 any
longer.  The only code fragment I've left in is support for
/proc/<pid/path/a.out in procfs.c (procfs_target::pid_to_exec_file):
while current Solaris 11 updates provide /proc/<pid>/execname, that
wasn't present in Solaris 11.0 and still isn't in current Illumos and I
didn't want to make live harder for them.

Tested on i386-pc-solaris2.10 (obsolete configuration rejected) and
x86_64-pc-linux-gnu x sparc64-solaris2.10 (likewise)
resp. x86_64-pc-linux-gnu x sparcv9-solaris2.11 (still builds; I'm using
the sparcv9 form for 64-bit SPARC customary on Solaris in the
MAINTAINERS file now).

	* NEWS (Changes since GDB 8.3): Document Solaris 10 removal.
	* configure.host: Mark *-*-solaris2.10* obsolete.
	* configure.tgt: Mark Solaris < 11 obsolete.
	* MAINTAINERS (Target Instruction Set Architectures) <sparc>:
	Update target triplet.