Fix darwin shared library and proc macro suffixes.

Darwin builds aren't using the correct suffix for shared libraries or
proc macros (.dylib). This fixes that.

Bug: 155302034
Test: "cd external/rust; mma" no longer indicates wrong extension.
Change-Id: I5093e0bcc78dbb2dd9fcf39069f326101bb8f411

rust/config/x86_darwin_host.go

@@ -62,7 +62,11 @@ func (t *toolchainDarwinX8664) RustTriple() string {
 	return "x86_64-apple-darwin"
 }
 
-func (t *toolchainDarwin) ShlibSuffix() string {
+func (t *toolchainDarwin) SharedLibSuffix() string {
+	return ".dylib"
+}
+
+func (t *toolchainDarwin) ProcMacroSuffix() string {
 	return ".dylib"
 }