Use new get_signal_number utility function for tests.

Summary:
This fixes TestLldbGdbServer and TestSendSignal from Windows to Android.
This change depends on D10171.

Reviewers: clayborg, ovyalov

Reviewed By: clayborg, ovyalov

Subscribers: tberghammer, lldb-commits

Differential Revision: http://reviews.llvm.org/D10172

llvm-svn: 238852