amdgcn: Support AMD-specific 'isa' traits in OpenMP context selectors

Add support for gfx803 as an alias for fiji.
Add test cases for all supported 'isa' values.

gcc/ChangeLog:

	* config/gcn/gcn.cc (gcn_omp_device_kind_arch_isa): Add gfx803.
	* config/gcn/t-omp-device: Add gfx803.

libgomp/ChangeLog:

	* testsuite/libgomp.c/declare-variant-4-fiji.c: New test.
	* testsuite/libgomp.c/declare-variant-4-gfx803.c: New test.
	* testsuite/libgomp.c/declare-variant-4-gfx900.c: New test.
	* testsuite/libgomp.c/declare-variant-4-gfx906.c: New test.
	* testsuite/libgomp.c/declare-variant-4-gfx908.c: New test.
	* testsuite/libgomp.c/declare-variant-4-gfx90a.c: New test.
	* testsuite/libgomp.c/declare-variant-4.h: New header file.