[compiler-rt] Allow appending to 'target_cflags' value from lit_config.

This patch is similar to D84708. When testing compiler-rt on different
baremetal targets, it helps to have the ability to pass some more parameters
at test time that allows you to build the test executable for a
given target. For an example, you may need a different linker command
file for different targets.

This patch will allows to do things like

$ llvm-lit --param=append_target_cflags="-T simulator.ld"
or
$ llvm-lit --param=append_target_cflags="-T hardware.ld"

In this way, you can run tests on different targets without having to run
cmake again.

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D91783