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.

std/build.rs

@@ -100,6 +100,8 @@ fn main() {
         ("x86" | "x86_64", _) => false,
         // Missing `__gnu_h2f_ieee` and `__gnu_f2h_ieee`
         ("powerpc" | "powerpc64", _) => false,
+        // Missing `__gnu_h2f_ieee` and `__gnu_f2h_ieee`
+        ("mips" | "mips32r6" | "mips64" | "mips64r6", _) => false,
         // Missing `__extendhfsf` and `__truncsfhf`
         ("riscv32" | "riscv64", _) => false,
         // Most OSs are missing `__extendhfsf` and `__truncsfhf`