Rollup merge of #61705 - petrhosek:llvm-cflags, r=alexcrichton
Rollup merge of #61705 - petrhosek:llvm-cflags, r=alexcrichton

Pass cflags rather than cxxflags to LLVM as CMAKE_C_FLAGS

We mistakenly pass cxxflags from the configuration to LLVM build as
CMAKE_C_FLAGS.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust