[runtime] Have the runtime use the compiler builtin for alloca on NetBSD (#73480)

Most of the tests were failing with the following in their logs..

| /usr/bin/ld: /home/brad/llvm-build/runtimes/runtimes-bins/openmp/runtime/src/libomp.so:
warning: Warning: reference to the libc supplied alloca(3); this most likely will not
work. Please use the compiler provided version of alloca(3), by supplying the appropriate
compiler flags (e.g. -std=gnu99).

By making use of __builtin_alloca..

before:

Total Discovered Tests: 353
  Unsupported:  59 (16.71%)
  Passed     :  51 (14.45%)
  Failed     : 243 (68.84%)

after:

Total Discovered Tests: 353
  Unsupported:  59 (16.71%)
  Passed     : 290 (82.15%)
  Failed     :   4 (1.13%)