[Flang][Test] Add support to change the default target triple for tests

In this patch I added support to change the default target triple used
by flang tests using the cmake variable: FLANG_TEST_TARGET_TRIPLE.

This functionality is implemented using the LLVM_TARGET_TRIPLE_ENV
variable, so that must be defined as well.

An example use:
`-DLLVM_TARGET_TRIPLE_ENV="LLVM_TARGET_TRIPLE_ENV" -DFLANG_TEST_TARGET_TRIPLE="aarch64-linux-gnu"`

Differential revision: https://reviews.llvm.org/D138530

Change-Id: I38e4a46a65109d415a9b72c8a0bf8a955e937280