cross-compile: riscv: look through whitelist of compilers

riscv currently has several competing compiler toolchain prefixes. If a
toolchain supports multilib, then an "incorrect" target triple may be
used.

For example, many distributions ship multilib toolchains with a
`riscv64` prefix, so a riscv32imac-unknown-none-elf target can
successfully use a toolchain with a triple of `riscv64-unknown-none`.

Signed-off-by: Sean Cross <sean@xobs.io>

src/lib.rs

@@ -2291,11 +2291,31 @@ impl Build {
             "powerpc-unknown-netbsd" => Some("powerpc--netbsd"),
             "powerpc64-unknown-linux-gnu" => Some("powerpc-linux-gnu"),
             "powerpc64le-unknown-linux-gnu" => Some("powerpc64le-linux-gnu"),
-            "riscv32i-unknown-none-elf" => Some("riscv32-unknown-elf"),
-            "riscv32imac-unknown-none-elf" => Some("riscv32-unknown-elf"),
-            "riscv32imc-unknown-none-elf" => Some("riscv32-unknown-elf"),
-            "riscv64gc-unknown-none-elf" => Some("riscv64-unknown-elf"),
-            "riscv64imac-unknown-none-elf" => Some("riscv64-unknown-elf"),
+            "riscv32i-unknown-none-elf" => self.find_working_gnu_prefix(&[
+                "riscv32-unknown-elf",
+                "riscv64-unknown-elf",
+                "riscv-none-embed",
+            ]),
+            "riscv32imac-unknown-none-elf" => self.find_working_gnu_prefix(&[
+                "riscv32-unknown-elf",
+                "riscv64-unknown-elf",
+                "riscv-none-embed",
+            ]),
+            "riscv32imc-unknown-none-elf" => self.find_working_gnu_prefix(&[
+                "riscv32-unknown-elf",
+                "riscv64-unknown-elf",
+                "riscv-none-embed",
+            ]),
+            "riscv64gc-unknown-none-elf" => self.find_working_gnu_prefix(&[
+                "riscv64-unknown-elf",
+                "riscv32-unknown-elf",
+                "riscv-none-embed",
+            ]),
+            "riscv64imac-unknown-none-elf" => self.find_working_gnu_prefix(&[
+                "riscv64-unknown-elf",
+                "riscv32-unknown-elf",
+                "riscv-none-embed",
+            ]),
             "riscv64gc-unknown-linux-gnu" => Some("riscv64-linux-gnu"),
             "s390x-unknown-linux-gnu" => Some("s390x-linux-gnu"),
             "sparc-unknown-linux-gnu" => Some("sparc-linux-gnu"),