ppc64le: Fix finding libclang_rt.builtins on Fedora\RHEL\CentOS

[mastertheknife]

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Changelog category (leave one):

• Build/Testing/Packaging Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Fix Fedora\RHEL\CentOS not finding libclang_rt.builtins on ppc64le
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Detailed description / Documentation draft:
This is because on Fedora\RHEL\CentOS this library has a different name. CMAKE_SYSTEM_PROCESSOR is ppc64le but the library is called powerpc64le. Clang has a nice feature for this. It can give libgcc or libclang_rt depending on the rtlib option.

[root@removed ~]# cmake --system-information | grep CMAKE_SYSTEM_PROCESSOR
CMAKE_SYSTEM_PROCESSOR "ppc64le"
[root@removed ~]# clang --print-file-name=libclang_rt.builtins-ppc64le.a
libclang_rt.builtins-ppc64le.a
[root@removed ~]# clang --print-libgcc-file-name --rtlib=compiler-rt
/usr/lib64/clang/10.0.1/lib/linux/libclang_rt.builtins-powerpc64le.a

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