fix compilation errors of MIPS target

This patch fixes some errors of MIPS target, however, MIPS C ABI is still broken. I will send another PR to fix the problem.

Because MIPS target has no "generic" CPU name, I add --target-cpu and --target-feature to RUST_FLAGS. In order to workaround the "compact frame descriptions incompatible with DWARF2 .eh_frame" problem, the linker I used is CXX but not CC.