std: Set has_reliable_f16 to false for MIPS targets in build.rs

To avoid this linker error:

    $ sudo apt install libc6-mips-cross gcc-mips-linux-gnu
    $ CC_mips_unknown_linux_gnu=mips-linux-gnu-gcc \
      CARGO_TARGET_MIPS_UNKNOWN_LINUX_GNU_LINKER=mips-linux-gnu-gcc \
      ./x test library/std --target mips-unknown-linux-gnu
    undefined reference to `__gnu_f2h_ieee'

You get the same linker error also with mipsel, mips64 and
mips64el toolchains.