rust: fix miscompilation of rustc on musl + enable vendor crates

This commit fixes several things.

1) It enables usage of vendor crates, making our patches actually
   do something (those for the vendor/ subdirectory anyway)
2) By extension, it enables dynamic linking of musl, which was not
   being done at all.
3) Removes the ugly LDFLAGS hack, which was causing miscompilation
   in cross environments, as it didn't pass the cross sysroot to
   search for libs in when linking. This resulted in a particularly
   bad miscompilation when cross-compiling from glibc to musl on
   the same architecture, as it would link against glibc's libc.a
   instead of musl's libc.a (not libc.so because the musl dynamic
   link patch was not being used), resulting in failures when
   actually compiling various projects. This hack is now replaced
   with properly patching the lzma-sys crate instead, which was
   the culprit previously when it comes to failures.

[ci skip]

srcpkgs/rust/patches/lzma-sys-cross-ldflags.patch

@@ -0,0 +1,11 @@
+This allows lzma-sys to build in a cross-compiling environment.
+--- rustc-1.33.0-src/vendor/lzma-sys/build.rs
++++ rustc-1.33.0-src/vendor/lzma-sys/build.rs
+@@ -101,6 +101,7 @@
+         }
+         cmd.env("CC", compiler.path())
+            .env("CFLAGS", cflags)
++           .env("LDFLAGS", "")
+            .current_dir(&dst.join("build"))
+            .arg(sanitize_sh(&src.join("configure")));
+         cmd.arg(format!("--prefix={}", sanitize_sh(&dst)));

srcpkgs/rust/template

@@ -1,7 +1,7 @@
 # Template file for 'rust'
 pkgname=rust
 version=1.34.0
-revision=2
+revision=3
 _rust_dist_version=1.33.0
 _cargo_dist_version=0.33.0
 # NB. if you push any(!) new version, don't forget to put a build
@@ -114,6 +114,13 @@ post_extract() {
 	fi
 }
 
+# we need this because cargo verifies checksums of all files in vendor
+# crates when it builds and gives us no way to override or update the
+# file sanely... so just clear out the file list
+_clear_vendor_checksums() {
+	sed -i 's/\("files":{\)[^}]*/\1/' vendor/$1/.cargo-checksum.json
+}
+
 post_patch() {
 	if [ "$build_option_internal_llvm" ]; then
 		# patches for Rust's bundled LLVM
@@ -133,6 +140,10 @@ post_patch() {
 	fi
 
 	sed -i /LD_LIBRARY_PATH/d src/bootstrap/bootstrap.py
+
+	# clear out all the checksum nonsense of patched vendor crates
+	_clear_vendor_checksums libc
+	_clear_vendor_checksums lzma-sys
 }
 
 do_configure() {
@@ -148,6 +159,7 @@ do_configure() {
 		--disable-rpath
 		--disable-docs
 		--disable-codegen-tests
+		--enable-vendor
 	"
 
 	if ! [ "$build_option_internal_llvm" ]; then
@@ -199,7 +211,7 @@ pre_build() {
 # of the cross host.
 # Unset LDFLAGS, otherwise cross builds to the same arch will fail
 do_build() {
-	env CFLAGS_${RUST_BUILD}="${CFLAGS_host}" LDFLAGS=''  make ${makejobs} ${make_build_args}
+	env CFLAGS_${RUST_BUILD}="${CFLAGS_host}" make ${makejobs} ${make_build_args}
 }
 
 do_install() {