libgomp: Fix on_device_arch.c aux-file handling [PR99555]

libgomp: Fix on_device_arch.c aux-file handling [PR99555]

libgomp/ChangeLog:

	PR target/99555
	* testsuite/lib/on_device_arch.c: Move to ...
	* testsuite/libgomp.c-c++-common/on_device_arch.h: ... here.
	* testsuite/libgomp.fortran/on_device_arch.c: New file;
	#include on_device_arch.h.
	* testsuite/libgomp.c-c++-common/task-detach-6.c: #include
	on_device_arch.h instead of using dg-additional-source.
	* testsuite/libgomp.c/pr99555-1.c: Likewise.
	* testsuite/libgomp.fortran/task-detach-6.f90: Update to use
	on_device_arch.c without relative paths.