The ARM linker will check that .o files declare compatible
build attributes (e.g., all claim hard-float calling convention,
all claim VFP-vX ,etc.). Thus, in order to set up cross tests that
link LLC generated code against and Subzero generated code,
we need the build attributes to be compatible.

Pick ARMv7, hard-float calling convention, and neon, etc. which
we use for PNaCl LLVM.

Will probably have to reorganize to keep in sync once the ELF
writer also emits this.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1171563002.