Make the default triple optional by allowing an empty string

When building LLVM as a (potentially dynamic) library that can be linked against
by multiple compilers, the default triple is not really meaningful.
We allow to explicitely set it to an empty string when configuring LLVM.
In this case, said "target independent" tests in the test suite that are using
the default triple are disabled by matching the newly available feature
"default_triple".

Reviewers: probinson, echristo
Differential Revision: http://reviews.llvm.org/D12660

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 247775
diff --git a/llvm/test/CodeGen/PowerPC/2006-12-07-SelectCrash.ll b/llvm/test/CodeGen/PowerPC/2006-12-07-SelectCrash.ll
index 9660d45..8536dda 100644
--- a/llvm/test/CodeGen/PowerPC/2006-12-07-SelectCrash.ll
+++ b/llvm/test/CodeGen/PowerPC/2006-12-07-SelectCrash.ll
@@ -1,6 +1,7 @@
 ; RUN: llc < %s -march=ppc64
 ; RUN: llc < %s -march=ppc32
 ; RUN: llc < %s
+; REQUIRES: default_triple
 
 @qsz.b = external global i1             ; <i1*> [#uses=1]