Subzero: Fix "make -f Makefile.standalone MINIMAL=1 check".

Subzero: Fix "make -f Makefile.standalone MINIMAL=1 check".

Some recent ARM changes turned out to break the lit tests for the MINIMAL build.  Two main issues:

1. ARM tests are currently asm-only, so allow_dump needs to be required.

2. GlobalContext::emitFileHeader() needs to return gracefully instead of calling report_fatal_error(), to allow error tests to produce the right error output.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1202563002