`x.py test` now requires Sparc target to be enabled in LLVM

[petrochenkov]

Reducing the number of built LLVM targets reduces time to build LLVM.
Previously targets = "X86;ARM" was enough to pass the testing successfully on a typical x86 machine, but recently one test (https://github.com/rust-lang/rust/blob/master/src/test/codegen/sparc-struct-abi.rs, cc @nagisa ) was added that requires enabling Sparc as well.

Target specific tests like this should preferably be somehow ignored if necessary LLVM targets are not built, or simply not included into the test suite.

[sanxiyn]

The test should be marked only-sparc64, which is the complement of ignore-sparc64.

[nagisa]

Hmm, only-sparc64 is a tad too restrictive than I’d like, but since I know of no other solution, lets just add that.