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

[Rainer Orth]

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).

Committed to trunk.

There are more instances of this issue: both libsanitizer and libgo
unnecessarily link with -lpthread, either unconditionally or due to a
configure test which doesn't check if the library is actually needed.
This can be fixed by consistently using AX_PTHREAD from
config/ax_pthread.m4, but such a fix affects all targets and is clearly
not stage 4 material.

	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University


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

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

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