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.
