llvm::orc::JITTargetMachineBuilder::detectHost() currently states:
// FIXME: getProcessTriple is bogus. It returns the host LLVM was compiled on,
//        rather than a valid triple for the current process.

This change uses LLVM_DEFAULT_TARGET_TRIPLE instead, which we define
based on the platform and predefined macros (see llvm-config.h).

This doesn't fix any known bugs, but it's good to not leave the triple
unknown, to avoid future issues. This change also refactors passing
the JITTargetMachineBuilder by rvalue reference and uses some less
cryptic variable names.

Bug: b/171236524
Change-Id: I7be752be66082654307210817250eb1dd025cb0c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49851Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>