compileopts: add CanonicalArchName to centralize arch detection

It's possible to detect the architecture from the target triple, but
there are a number of exceptions that make it unpleasant to use for this
purpose. There are just too many weird exceptions (like mips vs mipsel,
and armv6m vs thumv6m vs arm64 vs aarch64) so it's better to centralize
these to canonical architecture names.

I picked the architecture names that happen to match the musl
architecture names, because those seem the most natural to me.

builder/library.go

@@ -149,27 +149,24 @@ func (l *Library) load(config *compileopts.Config, tmpdir string) (job *compileJ
 	if config.ABI() != "" {
 		args = append(args, "-mabi="+config.ABI())
 	}
-	if strings.HasPrefix(target, "arm") || strings.HasPrefix(target, "thumb") {
+	switch compileopts.CanonicalArchName(target) {
+	case "arm":
 		if strings.Split(target, "-")[2] == "linux" {
 			args = append(args, "-fno-unwind-tables", "-fno-asynchronous-unwind-tables")
 		} else {
 			args = append(args, "-fshort-enums", "-fomit-frame-pointer", "-mfloat-abi=soft", "-fno-unwind-tables", "-fno-asynchronous-unwind-tables")
 		}
-	}
-	if strings.HasPrefix(target, "avr") {
+	case "avr":
 		// AVR defaults to C float and double both being 32-bit. This deviates
 		// from what most code (and certainly compiler-rt) expects. So we need
 		// to force the compiler to use 64-bit floating point numbers for
 		// double.
 		args = append(args, "-mdouble=64")
-	}
-	if strings.HasPrefix(target, "riscv32-") {
+	case "riscv32":
 		args = append(args, "-march=rv32imac", "-fforce-enable-int128")
-	}
-	if strings.HasPrefix(target, "riscv64-") {
+	case "riscv64":
 		args = append(args, "-march=rv64gc")
-	}
-	if strings.HasPrefix(target, "mips") {
+	case "mips":
 		args = append(args, "-fno-pic")
 	}