libgomp: Fix declare target link with offset array-section mapping [PR116107]

libgomp: Fix declare target link with offset array-section mapping [PR116107]

Assume that 'int var[100]' is 'omp declare target link(var)'. When now
mapping an array section with offset such as 'map(to:var[20:10])',
the device-side link pointer has to store &<device-storage-data>[0] minus
the offset such that var[20] will access <device-storage-data>[0]. But
the offset calculation was missed such that the device-side 'var' pointed
to the first element of the mapped data - and var[20] points beyond at
some invalid memory.

	PR middle-end/116107

libgomp/ChangeLog:

	* target.c (gomp_map_vars_internal): Honor array mapping offsets
	with declare-target 'link' variables.
	* testsuite/libgomp.c-c++-common/target-link-2.c: New test.