[BUG] Support for .cfi_startproc simple

Hi,

According to the GNU as documentation [1], .cfi_startproc simple is a
valid directive. Unfortunately, LLVM complains with:

  error: invalid instruction mnemonic 'simple'

I happened to notice it while attempting to compile linux.git with
clang: see arch/x86/ia32/ia32entry.S:75.

[1]: http://sourceware.org/binutils/docs-2.22/as/Pseudo-Ops.html#Pseudo-Ops

Thanks.

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