[compiler-rt] Allow nm program to be over-ridden for global symbol detection

[fjricci]

Summary

While cross-compiling, a custom nm program may be required. This will also allow for the
use of llvm-nm if desired.

[compnerd]

LGTM

compiler-rt/trunk/lib/sanitizer_common/scripts/gen_dynamic_list.py

versioned_functions = set(['memcpy', 'pthread_attr_getaffinity_np',
                            'pthread_cond_broadcast',
                            'pthread_cond_destroy', 'pthread_cond_init',
                            'pthread_cond_signal', 'pthread_cond_timedwait',
                            'pthread_cond_wait', 'realpath',
                            'sched_getaffinity'])
 
 def get_global_functions(library):
   functions = []
-  nm_proc = subprocess.Popen(['nm', library], stdout=subprocess.PIPE,
+  nm = os.environ.get('NM', 'nm')
+  nm_proc = subprocess.Popen([nm, library], stdout=subprocess.PIPE,
                              stderr=subprocess.PIPE)
   nm_out = nm_proc.communicate()[0].decode().split('\n')
   if nm_proc.returncode != 0:
-    raise subprocess.CalledProcessError(nm_proc.returncode, 'nm')
+    raise subprocess.CalledProcessError(nm_proc.returncode, nm)
   func_symbols = ['T', 'W']
   # On PowerPC, nm prints function descriptors from .data section.
   if os.uname()[4] in ["powerpc", "ppc64"]:
     func_symbols += ['D']
   for line in nm_out:
     cols = line.split(' ')
     if len(cols) == 3 and cols[1] in func_symbols :
       functions.append(cols[2])