Remove use_pthread option from getBuildFlags in the test environment

Removing the option to require the usage of pthread based on an argument
of the getBuildFlags method in lldbtest.py because it can be specified
by a variable in the makefile for the given test case. Using the
variable in the makefile works in all supported platforms.

Differential revision: http://reviews.llvm.org/D7861

llvm-svn: 230493