Work around ElementaryFunctions linker error on Ubuntu.

TF-892 tracks fixing the issue.

Author: dan-zheng

test/AutoDiff/differentiable_attr_cross_module/main.swift

@@ -2,7 +2,8 @@
 // modules.
 
 // RUN: %empty-directory(%t)
-// RUN: %target-build-swift %S/../Inputs/differentiable_attr_other_module.swift %s -o /dev/null
+// RUN: %target-build-swift %S/../Inputs/differentiable_attr_other_module.swift %s -o /dev/null -lm
+// NOTE(TF-892): `-lm` is necessary to prevent linker errors related to `ElementaryFunctions` on Ubuntu.
 
 @differentiable(wrt: x)
 func testInitializer(_ x: Float) -> Float {