Remove obsolete mentions of LLVM_HOST_BUILD_MK

The libbacktrace common flags don't need the flags from
LLVM_HOST_BUILD_MK, they are added directly to libbacktrace_offline.
The -fno-omit-frame-pointer is unnecessary, as we no longer import
flags only used to build LLVM into everything that depends on LLVM.
Removing -Wno-extern-c-compat doesn't cause any warnings.

Test: mma -j
Change-Id: Ia01380e8572c3d84268843291a54ec8c932a327f