d: Merge upstream dmd 934df6f8c, druntime 7bdd83d7

Corrects a previous change made to the SPARC stdc bindings, and
backports PPC-related fixes.  The library and language testsuite now
passes fully on powerpc64le-linux-gnu.

Fixes: PR d/90719
Fixes: PR d/94825

Reviewed-on: https://github.com/dlang/dmd/pull/11079
	     https://github.com/dlang/druntime/pull/3078
	     https://github.com/dlang/druntime/pull/3083

libphobos/ChangeLog:

	PR d/94825
	* libdruntime/Makefile.am (DRUNTIME_SOURCES_CONFIGURED): Remove
	config/powerpc/switchcontext.S
	* libdruntime/Makefile.in: Regenerate.
	* libdruntime/config/powerpc/callwithstack.S: Remove.
	* libdruntime/config/powerpc/switchcontext.S: Fix symbol name of
	fiber_switchContext.
	* libdruntime/core/thread.d: Disable fiber migration tests on PPC.
	* testsuite/libphobos.thread/fiber_guard_page.d: Set guardPageSize
	same as stackSize.