On Linux, clear the signal mask of the launched inferior

Summary:
Due to fork()/execve(), the launched inferior inherits the signal mask of its parent (lldb-server). But because lldb-server modifies its signal mask (It blocks SIGCHLD, for example), the inferior starts with some signals being initially blocked.

One consequence is that TestCallThatRestarts.ExprCommandThatRestartsTestCase (test/expression_command/call-restarts) fails because sigchld_handler() in lotta-signals.c is not called, due to the SIGCHLD signal being blocked.

To prevent the signal masking done by lldb-server from affecting the created inferior, the signal mask of the inferior is now cleared before the execve().

Patch by: Yacine Belkadi

Reviewers: ovyalov, labath

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12138

llvm-svn: 245436