[Libomptarget] Increase stack size for bug49779 test

The 'bug49779.cpp' test has been failing recently. This is because the
runtime is sufficiently complex when using nested parallelism without
optimizations that the CUDA tools cannot statically determine the stack
size. Because of this the kernel can exceed the thread stack size and
crash. Work around this using the 'LIBOMPTARGET_STACK_SIZE' environment
variable and add an FAQ entry for this situation.

Fixes #53670

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D119357