gn build: Fix redundant object files in builtin lib.

compiler-rt's builtin library has generic implementations of many
functions, and then per-arch optimized implementations of some.

In the CMake build, both filter_builtin_sources() and an explicit loop
at the end of the build file (see D37166) filter out the generic
versions if a per-arch file is present.

The GN build wasn't doing this filtering. Just do the filtering manually
and explicitly, instead of being clever.

While here, also remove files from the mingw/arm build that are
redundantly listed after D39938 / r318139 (both from the CMake and the
GN build).

While here, also fix a target_os -> target_cpu typo.

Differential Revision: https://reviews.llvm.org/D65512

llvm-svn: 367448