config: -pthread shouldn't link with -lpthread on Solaris

config: -pthread shouldn't link with -lpthread on Solaris

libpthread has been folded into libc since Solaris 10 and replaced by a
filter on libc.  Linking with libpthread thus only creates unnecessary
runtime overhead.

This patch thus removes linking with -lpthread if -pthread/-pthreads is
specified, thus getting rid of the libpthread dependency in libatomic,
libgdruntime, libgomp, libgphobos, and libitm.

Bootstrapped without regressions on i386-pc-solaris2.11 and
sparc-sun-solaris2.11 (both Solaris 11.3 and 11.4).

2023-04-03  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	* config/sol2.h (LIB_SPEC): Don't link with -lpthread.