[lldb] Update TestSimulatorPlatform.py to set ARCH_CFLAGS instead of TRIPLE
I move the triple (de)composition logic into the builder in e5d08fcbac72
but this test is relying on Make to construct the set the ARCH,
ARCH_CFLAGS and SDKROOT based on the given TRIPLE. This patch updates
the test to pass these variables directly.
Differential revision: https://reviews.llvm.org/D86244