Mostly to make them a bit more portable across OSes.
Otherwise the OS assumed by llvm-mc is the build/host OS. So,
on Mac llvm-mc will assume it's targeting darwin and only accepts macho
assembler directives. Assembler directives like .rodata.cst8 are not accepted
(I'm guessing it uses .cstring, .literal4, etc. instead?).

Force an OS (NaCl) so that ELF-related assembler macros make sense.

Also remove a now unused function typeIdentString to make clang happy.

Example errors:
Command 5 Stderr:
<stdin>:5:2: error: unknown directive
        .type   fixed_400,@function
        ^
<stdin>:23:2: error: unknown directive
        .type   variable_n,@function
        ^
<stdin>:40:11: error: mach-o section specifier uses an unknown section type
        .section        .rodata.cst4,"aM",@progbits,4
                        ^
<stdin>:42:11: error: mach-o section specifier uses an unknown section type
        .section        .rodata.cst8,"aM",@progbits,8
                        ^

BUG=none
R=stichnot@chromium.org, wala@chromium.org

Review URL: https://codereview.chromium.org/467103004