[test] Fix BugPoint/compile-custom.ll to use detected python exec

[mgorny]

Summary

Spawn the custom compile command in BugPoint/compile-custom.ll via
%python rather than relying on implicit 'env python' shebang, in order
to fix it on systems that don't have 'python' executable such as NetBSD.

llvm/trunk/test/BugPoint/compile-custom.ll

-; RUN: bugpoint -load %llvmshlibdir/BugpointPasses%shlibext --compile-custom --compile-command="%/s.py arg1 arg2" --opt-command opt --output-prefix %t %s | FileCheck %s
+; RUN: bugpoint -load %llvmshlibdir/BugpointPasses%shlibext --compile-custom --compile-command="%python %/s.py arg1 arg2" --opt-command opt --output-prefix %t %s | FileCheck %s
 ; REQUIRES: loadable_module
 
 ; Test that arguments are correctly passed in --compile-command.  The output
 ; of bugpoint includes the output of the custom tool, so we just echo the args
 ; in the tool and check here.
 
 ; CHECK: Error: arg1 arg2
 
 define void @noop() {
     ret void
 }