Add MIPS asm! support

This patch also:
* Add soft-float supports: only f32
* zero-extend i8/i16 to i32 because MIPS only supports register-length
  arithmetic.
* Update table in asm! chapter in unstable book.

compiler/rustc_target/src/asm/mips.rs

@@ -0,0 +1,132 @@
+use super::{InlineAsmArch, InlineAsmType};
+use rustc_macros::HashStable_Generic;
+use std::fmt;
+
+def_reg_class! {
+    Mips MipsInlineAsmRegClass {
+        reg,
+        freg,
+    }
+}
+
+impl MipsInlineAsmRegClass {
+    pub fn valid_modifiers(self, _arch: super::InlineAsmArch) -> &'static [char] {
+        &[]
+    }
+
+    pub fn suggest_class(self, _arch: InlineAsmArch, _ty: InlineAsmType) -> Option<Self> {
+        None
+    }
+
+    pub fn suggest_modifier(
+        self,
+        _arch: InlineAsmArch,
+        _ty: InlineAsmType,
+    ) -> Option<(char, &'static str)> {
+        None
+    }
+
+    pub fn default_modifier(self, _arch: InlineAsmArch) -> Option<(char, &'static str)> {
+        None
+    }
+
+    pub fn supported_types(
+        self,
+        _arch: InlineAsmArch,
+    ) -> &'static [(InlineAsmType, Option<&'static str>)] {
+        match self {
+            Self::reg => types! { _: I8, I16, I32, F32; },
+            Self::freg => types! { _: F32; },
+        }
+    }
+}
+
+// The reserved registers are somewhat taken from <https://git.io/JUR1k#L150>.
+def_regs! {
+    Mips MipsInlineAsmReg MipsInlineAsmRegClass {
+        v0: reg = ["$2", "$v0"],
+        v1: reg = ["$3", "$v1"],
+        a0: reg = ["$4", "$a0"],
+        a1: reg = ["$5", "$a1"],
+        a2: reg = ["$6", "$a2"],
+        a3: reg = ["$7", "$a3"],
+        // FIXME: Reserve $t0, $t1 if in mips16 mode.
+        t0: reg = ["$8", "$t0"],
+        t1: reg = ["$9", "$t1"],
+        t2: reg = ["$10", "$t2"],
+        t3: reg = ["$11", "$t3"],
+        t4: reg = ["$12", "$t4"],
+        t5: reg = ["$13", "$t5"],
+        t6: reg = ["$14", "$t6"],
+        t7: reg = ["$15", "$t7"],
+        s0: reg = ["$16", "$s0"],
+        s1: reg = ["$17", "$s1"],
+        s2: reg = ["$18", "$s2"],
+        s3: reg = ["$19", "$s3"],
+        s4: reg = ["$20", "$s4"],
+        s5: reg = ["$21", "$s5"],
+        s6: reg = ["$22", "$s6"],
+        s7: reg = ["$23", "$s7"],
+        t8: reg = ["$24", "$t8"],
+        t9: reg = ["$25", "$t9"],
+        f0: freg = ["$f0"],
+        f1: freg = ["$f1"],
+        f2: freg = ["$f2"],
+        f3: freg = ["$f3"],
+        f4: freg = ["$f4"],
+        f5: freg = ["$f5"],
+        f6: freg = ["$f6"],
+        f7: freg = ["$f7"],
+        f8: freg = ["$f8"],
+        f9: freg = ["$f9"],
+        f10: freg = ["$f10"],
+        f11: freg = ["$f11"],
+        f12: freg = ["$f12"],
+        f13: freg = ["$f13"],
+        f14: freg = ["$f14"],
+        f15: freg = ["$f15"],
+        f16: freg = ["$f16"],
+        f17: freg = ["$f17"],
+        f18: freg = ["$f18"],
+        f19: freg = ["$f19"],
+        f20: freg = ["$f20"],
+        f21: freg = ["$f21"],
+        f22: freg = ["$f22"],
+        f23: freg = ["$f23"],
+        f24: freg = ["$f24"],
+        f25: freg = ["$f25"],
+        f26: freg = ["$f26"],
+        f27: freg = ["$f27"],
+        f28: freg = ["$f28"],
+        f29: freg = ["$f29"],
+        f30: freg = ["$f30"],
+        f31: freg = ["$f31"],
+        #error = ["$0", "$zero"] =>
+            "constant zero cannot be used as an operand for inline asm",
+        #error = ["$1", "$at"] =>
+            "reserved for assembler (Assembler Temp)",
+        #error = ["$26", "$k0"] =>
+            "OS-reserved register cannot be used as an operand for inline asm",
+        #error = ["$27", "$k1"] =>
+            "OS-reserved register cannot be used as an operand for inline asm",
+        #error = ["$28", "$gp"] =>
+            "the global pointer cannot be used as an operand for inline asm",
+        #error = ["$29", "$sp"] =>
+            "the stack pointer cannot be used as an operand for inline asm",
+        #error = ["$30", "$s8", "$fp"] =>
+            "the frame pointer cannot be used as an operand for inline asm",
+        #error = ["$31", "$ra"] =>
+            "the return address register cannot be used as an operand for inline asm",
+    }
+}
+
+impl MipsInlineAsmReg {
+    pub fn emit(
+        self,
+        out: &mut dyn fmt::Write,
+        _arch: InlineAsmArch,
+        _modifier: Option<char>,
+    ) -> fmt::Result {
+        out.write_str(self.name())
+    }
+}