illumos gate

Build errors with gcc 9:

    In file included from picldevtree.c:40:
    picldevtree.c: In function 'get_first_reg_word':
    /code/illumos-gate/proto/root_i386/usr/include/alloca.h:57:19: error: argument to 'alloca' may be too large [-Werror=alloca-larger-than=]
       57 | #define alloca(x) __builtin_alloca(x)
          |                   ^~~~~~~~~~~~~~~~~~~
    picldevtree.c:2962:11: note: in expansion of macro 'alloca'
     2962 |  regbuf = alloca(pinfo.piclinfo.size);
          |           ^~~~~~
    cc1: all warnings being treated as errors

Use heap allocations instead.

Testing done: build/install/boot.