Fix group stop signal handling issue on Linux.

This patch addresses a bug where in a multi-threaded program a new
signal from the inferior may be received before all group-stop
messages from an earlier signal have been handled.

Patch by Andrew MacPherson
MC: Add support for .cfi_startproc simple

This commit allows LLVM MC to process .cfi_startproc directives when
they are followed by an additional `simple' identifier. This signals to
elide the emission of target specific CFI instructions that would
normally occur initially.

This fixes PR16587.

Differential Revision: http://llvm-reviews.chandlerc.com/D2624