[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