GCN: Generally enable the 'gcc.target/gcn/avgpr-[...]' test cases
... added in commit
ae0d2c240213c5a7f6959c032bfc9f0703cab787
"amdgcn: Add Accelerator VGPR registers". This way, they're correctly tested
no matter what '-march=[...]' is used with 'make check'.
gcc/testsuite/
* gcc.target/gcn/avgpr-mem-double.c: Remove
'dg-skip-if "incompatible ISA" [...]'.
* gcc.target/gcn/avgpr-mem-int.c: Likewise.
* gcc.target/gcn/avgpr-mem-long.c: Likewise.
* gcc.target/gcn/avgpr-mem-short.c: Likewise.
* gcc.target/gcn/avgpr-spill-double.c: Likewise.
* gcc.target/gcn/avgpr-spill-int.c: Likewise.
* gcc.target/gcn/avgpr-spill-long.c: Likewise.
* gcc.target/gcn/avgpr-spill-short.c: Likewise.