llvm-rc: Enable preprocessing if CMAKE_RC_COMPILER_INIT is an absolute path

WangBinrequested to merge
wang-bin/cmake:llvm_rc_pp into master

CMAKE_RC_COMPILER_INIT and CMAKE_RC_COMPILER can be set by user, for example llvm-rc-10 and D:\LLVM\bin\llvm-rc.exe.

Backport: release

Edited by Brad King

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