From: Sam James <sam@gentoo.org>
Subject: Allow RUST_TARGET override for detected Rust triplet (fixes musl build)

Respect RUST_TARGET from the environment to override Mozilla's
own triplet detection.

If the variable is unset, the old logic is used.

The issue is that Gentoo supports both, say:
- x86_64-unknown-linux-musl, and
- x86-64-gentoo-linux-musl

Our Rust is built to understand
-gentoo- as a vendor target, but
our Rust only actually includes
-unknown-.

Mozilla's configure sees -gentoo-
in CHOST and that Rust _understands_
-gentoo- and tries to use it, without
verifying that we actually have
-gentoo- installed as a Rust target.

This requires exporting RUST_TARGET
in the ebuild, possibly like:
````
inherit rust-toolchain

src_configure() {
	[...]
	export RUST_TARGET=$(rust_abi)
	[...]
}
````

Inspired by Alpine's patch at
https://git.alpinelinux.org/aports/tree/community/firefox/fix-rust-target.patch?id=740922900cf8042be6751fcfcccdd3fc11bd77d3.

Bug: https://bugs.gentoo.org/748849
Bug: https://bugs.gentoo.org/779178
Bug: https://bugs.gentoo.org/836226
Thanks-to: Georgy Yakovlev <gyakovlev@gentoo.org>
--- a/build/moz.configure/rust.configure
+++ b/build/moz.configure/rust.configure
@@ -275,6 +275,7 @@ def rust_supported_targets(rustc):
     return data
 
 
+@imports('os')
 def detect_rustc_target(
     host_or_target, compiler_info, arm_target, rust_supported_targets
 ):
@@ -396,7 +397,7 @@ def detect_rustc_target(
 
         return None
 
-    rustc_target = find_candidate(candidates)
+    rustc_target = os.environ.get('RUST_TARGET', find_candidate(candidates))
 
     if rustc_target is None:
         die("Don't know how to translate {} for rustc".format(host_or_target.alias))