Add support of MIPS r6, o32 and n64

MIPS r6 uses new triples,
  mipsisa{32,64}r6{el,}-linux-gnu{,abi64}

The support of these triples has been in rust main project.

Author: wzssyqa

src/lib.rs

@@ -1787,6 +1787,10 @@ impl Build {
                         "mipsel-unknown-linux-gnu" => Some("mipsel-linux-gnu"),
                         "mips64-unknown-linux-gnuabi64" => Some("mips64-linux-gnuabi64"),
                         "mips64el-unknown-linux-gnuabi64" => Some("mips64el-linux-gnuabi64"),
+                        "mipsisa32r6-unknown-linux-gnu" => Some("mipsisa32r6-linux-gnu"),
+                        "mipsisa32r6el-unknown-linux-gnu" => Some("mipsisa32r6el-linux-gnu"),
+                        "mipsisa64r6-unknown-linux-gnuabi64" => Some("mipsisa64r6-linux-gnuabi64"),
+                        "mipsisa64r6el-unknown-linux-gnuabi64" => Some("mipsisa64r6el-linux-gnuabi64"),
                         "powerpc-unknown-linux-gnu" => Some("powerpc-linux-gnu"),
                         "powerpc-unknown-linux-gnuspe" => Some("powerpc-linux-gnuspe"),
                         "powerpc-unknown-netbsd" => Some("powerpc--netbsd"),