libphobos: Enable on Solaris/SPARC or with /bin/as [PR 103528]

libphobos is currently only enabled on Solaris/x86 with gas.  As
discovered when gdc was switched to the dmd frontend, this initially
broke bootstrap for the other Solaris configurations.

However, it's now well possible to enable it both for Solaris/x86 with
as and Solaris/SPARC (both as and gas) since the original problems (x86
as linelength limit, among others) are long gone.

The following patch does just that.

Tested on i386-pc-solaris2.11 and sparc-sun-solaris2.11 (both as and
gas) with gdc 9.3.0 (x86) resp. 9.4.0 (sparc, configured with
--enable-libphobos) as bootstrap compilers.

2021-12-01  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	libphobos:
	PR d/103528
	* configure.ac <x86_64-*-solaris2.* | i?86-*-solaris2.*>: Remove
	gas requirement.
	* configure: Regenerate.
	* configure.tgt (sparc*-*-solaris2.11*): Mark supported.