It's been a few months since I went down this rabbit hole, but IIRC, it worked a little better to build clang on ppc directly. 10.5 intel fails to build PPC correctly due to the runtime library issues you stumbled across. The highest i have been able to go is clang-3.6 on ppc so far (building it on PPC, using clang-3.4). clang-3.7+ will not build successfully, although llvm-3.7+ does build (wouldn't trust the code it produces on PPC, though).
Libc++ works, but the exceptions don't work correctly still. But clang > 3.4 works in a spotty fashion, and the produced binaries have errors. llvm > 3.4 produces buggy ppc code that crashes easily. I might hope to find a way to get clang-3.7 to output intermediate code that gcc5 or gcc6 might further process (AKA llvm-4.2), but I guess that seems pretty unlikely these days. here's what I have working on 10.5 ppc at present: LeopardG5:MacPorts-2.4.1 macportsdev$ port -v installed | grep clang clang-3.4 @3.4.2_12+analyzer platform='darwin 9' archs='ppc' date='2016-11-08T21:25:19-0800' clang-3.6 @3.6.2_5+analyzer platform='darwin 9' archs='ppc' date='2016-12-29T22:18:32-0800' clang_select @2_0 (active) platform='darwin 9' archs='noarch' date='2016-10-02T00:17:54-0700' LeopardG5:MacPorts-2.4.1 macportsdev$ port -v installed | grep llvm cctools @886_6+llvm33 (active) platform='darwin 9' archs='ppc' date='2016-10-22T10:49:21-0700' ld64-127 @127.2_8+llvm33 (active) platform='darwin 9' archs='ppc' date='2016-11-08T21:00:26-0800' llvm-3.3 @3.3_10 (active) platform='darwin 9' archs='ppc' date='2016-10-02T17:27:59-0700' llvm-3.4 @3.4.2_11 (active) platform='darwin 9' archs='ppc' date='2016-10-02T17:32:34-0700' llvm-3.6 @3.6.2_4 (active) platform='darwin 9' archs='ppc' date='2016-12-29T19:47:09-0800' llvm-3.8 @3.8.1_0 (active) platform='darwin 9' archs='ppc' date='2017-01-06T17:43:28-0800' llvm_select @2_0 (active) platform='darwin 9' archs='noarch' date='2016-10-02T00:18:02-0700' LeopardG5:MacPorts-2.4.1 macportsdev$ port -v installed | grep libc libcxx @3.9.0_0+universal (active) platform='darwin 9' archs='i386 ppc x86_64' date='2016-12-11T15:19:16-0800' libcxxabi @3.9.0_0+universal (active) platform='darwin 9' archs='i386 ppc x86_64' date='2016-12-11T15:17:25-0800'
