doc: Update install.texi for GCC 10 on Solaris

doc: Update install.texi for GCC 10 on Solaris

This patch updates install.texi for GCC 10 on Solaris.  It includes some
general cleanup and updates and includes a couple of caveats, some of
them found when testing GCC 10.1.0 RC1 with only the bundled tools.

The reference to TGCware on the binaries page is gone because they only
provide binaries up to Solaris 9.

The note about configuring 64-bit Solaris/SPARC compilers with
--build=sparcv9-sun-solaris2.11 became necessary because upstream
config.guess silently reverted my patch to automatically detect this.  We
already had a bug report about a comparison failure caused by this issue.

Tested with make doc/gccinstall.info and doc/gccinstall.pdf and
inspection of the resulting files.

	* doc/install.texi: Replace Sun with Solaris as appropriate.
	(Tools/packages necessary for building GCC, Perl version between
	5.6.1 and 5.6.24): Remove Solaris 8 reference.
	(Installing GCC: Binaries, Solaris 2 (SPARC, Intel)): Remove
	TGCware reference.
	(Specific, i?86-*-solaris2*): Update version references for
	Solaris 11.3 and later.  Remove gas 2.26 caveat.
	(Specific, *-*-solaris2*): Update version references for
	Solaris 11.3 and later.  Remove boehm-gc reference.
	Document GMP, MPFR caveats on Solaris 11.3.
	(Specific, sparc-sun-solaris2*): Update Solaris 9 references.
	(Specific, sparc64-*-solaris2*): Likewise.
	Document --build requirement.