[PowerPC] Fix check for ieeelongdouble support

Clang detects the GCC version from the libdir. However, modern
GCC versions only include the major version in the libdir
(something like lib/gcc/powerpc64le-linux-gnu/12/), not all
version components. For this reason, even though the system has
a supported libstdcxx, it will still fail the check against the
12.1.0 version requirement.

Fix this by doing the same thing we do for patch versions: Assume
that a missing minor version is larger than any specific version.

To allow this to be tested, we need to fix two additional issues:
First, the GCC toolchain directories used for testing need to
contain a crtbegin.o file to be properly detected. The existing
tests actually ended up using a 0.0.0 version, rather the intended
one. Second, we also need to satisfy the glibc version check based
on the dynamic linker. To do so, respect the --dyld-prefix argument
and add the necessary file to the test toolchain directory.

Differential Revision: https://reviews.llvm.org/D136258

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -1884,8 +1884,15 @@ bool Generic_GCC::GCCVersion::isOlderThan(int RHSMajor, int RHSMinor,
                                           StringRef RHSPatchSuffix) const {
   if (Major != RHSMajor)
     return Major < RHSMajor;
-  if (Minor != RHSMinor)
+  if (Minor != RHSMinor) {
+    // Note that versions without a specified minor sort higher than those with
+    // a minor.
+    if (RHSMinor == -1)
+      return true;
+    if (Minor == -1)
+      return false;
     return Minor < RHSMinor;
+  }
   if (Patch != RHSPatch) {
     // Note that versions without a specified patch sort higher than those with
     // a patch.

clang/lib/Driver/ToolChains/PPCLinux.cpp

@@ -82,6 +82,7 @@ bool PPCLinuxToolChain::SupportIEEEFloat128(
       (StdLib == CST_Libstdcxx &&
        GCCInstallation.getVersion().isOlderThan(12, 1, 0));
 
-  return GlibcSupportsFloat128(Linux::getDynamicLinker(Args)) &&
+  std::string Linker = Linux::getDynamicLinker(Args);
+  return GlibcSupportsFloat128((Twine(D.DyldPrefix) + Linker).str()) &&
          !(D.CCCIsCXX() && HasUnsupportedCXXLib);
 }

clang/test/Driver/ppc-float-abi-warning.cpp

@@ -3,6 +3,11 @@
 // RUN:  --gcc-toolchain=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-11.2.0 \
 // RUN:  -mabi=ieeelongdouble -stdlib=libstdc++ 2>&1 | FileCheck %s
 // RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s \
+// RUN:  --gcc-toolchain=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-12 \
+// RUN:  --dyld-prefix=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-12 \
+// RUN:  -mabi=ieeelongdouble -stdlib=libstdc++ 2>&1 | \
+// RUN:  FileCheck %s --check-prefix=NOWARN
+// RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s \
 // RUN:  -mabi=ieeelongdouble -stdlib=libc++ 2>&1 | FileCheck %s
 // RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s\
 // RUN:  -mabi=ieeelongdouble -stdlib=libc++ -Wno-unsupported-abi 2>&1 | \