pnacl-translate: Refactor selection of arch-specific flags
pnacl-translate: Refactor selection of arch-specific flags

This removes uses of the messy "${FOO_%ARCH%}" construct in
pnacl-translate.py and replaces these uses with Python code.

This makes it easier to reduce duplication.  Otherwise, the
duplication would increase when we add {x86-64,arm,mips32}-nonsfi
targets and extend unsandboxed mode to work for Mac, Android, Windows,
etc.

I've removed a special case for MIPS: "-translate-fast" was using
"-fast-isel" on MIPS but "-O0" elsewhere.  I don't know the reason for
this difference, and it wasn't commented.  I can't see anything in the
LLVM codebase that suggests that "-fast-isel" is special for MIPS.
Removing this difference only affects offline translation, and I
assume it's not important (though it does seem to change the generated
code).

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3699
TEST=toolchain trybots
R=jvoung@chromium.org, petarj@mips.com

Review URL: https://codereview.chromium.org/214213004

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12971 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client