Fix architecture detection on ppc64le

[taylordotfish]

juce_runtime_arch_detection.cpp currently identifies ppc64le as ppc64, which causes JUCE to use the directory name ppc64-linux for VST 3 plugin contents. However, VST 3 specifies that uname -m should be used as the first component of the directory name, which on 64-bit little-endian PowerPC is ppc64le.

Currently, this causes problems when building VST 3 plugins on this platform, as the VST 3 SDK expects the module directory to be named ppc64le-linux.

This PR adds an additional endianness check when 64-bit PowerPC is detected, outputting ppc64 or ppc64le as appropriate.