Avoid SIGSEGV in nvptx 'mkoffload' for voluminous PTX code

Avoid SIGSEGV in nvptx 'mkoffload' for voluminous PTX code

In commit 50be486dff4ea2676ed022e9524ef190b92ae2b1
"nvptx: libgomp+mkoffload.cc: Prepare for reverse offload fn lookup", some
additional tracking of the PTX code was added, and this assumes that
potentially every single character of PTX code needs to be tracked as a new
chunk of PTX code.  That's problematic if we're dealing with voluminous PTX
code (for example, non-trivial C++ code), and the 'file_idx' 'alloca'tion then
causes stack overflow.  For example:

    FAIL: libgomp.c++/target-std__valarray-1.C (test for excess errors)
    UNRESOLVED: libgomp.c++/target-std__valarray-1.C compilation failed to produce executable

    lto-wrapper: fatal error: [...]/build-gcc/gcc//accel/nvptx-none/mkoffload terminated with signal 11 [Segmentation fault], core dumped

	gcc/
	* config/nvptx/mkoffload.cc (process): Use an 'auto_vec' for
	'file_idx'.