[compiler-rt] Allow override of 'emulator' value from lit_config.

Currently the 'emulator' value is fixed at build time. This patch allows changing the emulator
at testing time and enables us to run the tests on different board or simulators without needing
to run CMake again to change the value of emulator.

With this patch in place, the value of 'emulator' can be changed at test time from the command
line like this:

$ llvm-lit --param=emulator="..."

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D84708