Make the jit work with a cg_llvm libstd

[bjorn3]

• Don't try to load the rust_metadata_* symbol, which is part of a section without the LOAD flag. (25f7eee)
• Avoid attempted re-initialization of the current thread's ThreadInfo when the same libstd is used by rustc and the jitted program. (e86b954)
• compiler_builtins intrinsics not exported from libstd.so. (94b51d1)
• cg_llvm's libstd.so is compiled with -Cpanic=unwind, while cg_clif always uses -Cpanic=abort.
• Ensure that is_x86_feature_detected returns false for features unsupported by cg_clif. (optional. only necessary for crates using SIMD) is_x86_feature_detected is not longer being patched.