nvptx: Delay 'sorry, unimplemented: dynamic stack allocation not supported'...

nvptx: Delay 'sorry, unimplemented: dynamic stack allocation not supported' from expansion time to code generation

This gives the back end a chance to clean out a few more unnecessary instances
of dynamic stack allocation.  This progresses:

    PASS: gcc.dg/pr78902.c  (test for warnings, line 7)
    PASS: gcc.dg/pr78902.c  (test for warnings, line 8)
    PASS: gcc.dg/pr78902.c  (test for warnings, line 9)
    PASS: gcc.dg/pr78902.c  (test for warnings, line 10)
    PASS: gcc.dg/pr78902.c  (test for warnings, line 11)
    PASS: gcc.dg/pr78902.c  (test for warnings, line 12)
    PASS: gcc.dg/pr78902.c  (test for warnings, line 13)
    PASS: gcc.dg/pr78902.c strndup excessive bound at line 14 (test for warnings, line 13)
    [-UNSUPPORTED: gcc.dg/pr78902.c: dynamic stack allocation not supported-]
    {+PASS: gcc.dg/pr78902.c (test for excess errors)+}

    UNSUPPORTED: gcc.dg/torture/pr71901.c   -O0 : dynamic stack allocation not supported
    [-UNSUPPORTED:-]{+PASS:+} gcc.dg/torture/pr71901.c   -O1  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}
    UNSUPPORTED: gcc.dg/torture/pr71901.c   -O2 : dynamic stack allocation not supported
    UNSUPPORTED: gcc.dg/torture/pr71901.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions : dynamic stack allocation not supported
    UNSUPPORTED: gcc.dg/torture/pr71901.c   -O3 -g : dynamic stack allocation not supported
    [-UNSUPPORTED:-]{+PASS:+} gcc.dg/torture/pr71901.c   -Os  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}

    UNSUPPORTED: gcc.dg/torture/pr78742.c   -O0 : dynamic stack allocation not supported
    [-UNSUPPORTED:-]{+PASS:+} gcc.dg/torture/pr78742.c   -O1  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}
    [-UNSUPPORTED:-]{+PASS:+} gcc.dg/torture/pr78742.c   -O2  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}
    [-UNSUPPORTED:-]{+PASS:+} gcc.dg/torture/pr78742.c   -O3 -fomit-frame-pointer -funroll-loops -fpeel-loops -ftracer -finline-functions  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}
    [-UNSUPPORTED:-]{+PASS:+} gcc.dg/torture/pr78742.c   -O3 -g  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}
    UNSUPPORTED: gcc.dg/torture/pr78742.c   -Os : dynamic stack allocation not supported

    [-UNSUPPORTED:-]{+PASS:+} gfortran.dg/pr101267.f90   -O  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}

    [-UNSUPPORTED:-]{+PASS:+} gfortran.dg/pr112404.f90   -O  [-: dynamic stack allocation not supported-]{+(test for excess errors)+}

	gcc/
	* config/nvptx/nvptx.md (define_expand "allocate_stack")
	[!TARGET_SOFT_STACK]: Move
	'sorry ("dynamic stack allocation not supported");'...
	(define_insn "@nvptx_alloca_<mode>"): ... here.
	gcc/testsuite/
	* gcc.target/nvptx/alloca-1-unused-O0-sm_30.c: Adjust.