Prevent clang from using the x18 register on arm64.

This is achieved via the compiler flag -ffixed-x18 for non-LTO builds
and the linker flag -plugin-opt -mattr=+reserve-x18 for LTO builds.

This change should be reverted once we upgrade past LLVM r340889
which does this by default on Android.

Bug: 112907825
Bug: 111759196
Change-Id: I05473ddbb98319d87d442425b4d715647eae3a38