arch: Correct march to mcpu for ppc

Switch between -march=native (x86) and -mcpu=native -mtune=native (PowerPC)

Costmetic changes so `mcpu` and `march` remain the first flag

Fix indentation

arch: Correct march to mcpu for ppc

Correcting commit message since editing the title on github webui wasn't enough (I couldn't figure out how to rebase from the webui, sorry!).

Maybe you can squash to this commit when merging?

devito/arch/compiler.py

@@ -386,7 +386,7 @@ def __init__(self, *args, **kwargs):
 
         platform = kwargs.pop('platform', configuration['platform'])
 
-        self.cflags += ['-march=native', '-Wno-unused-result',
+        self.cflags += ['-Wno-unused-result',
                         '-Wno-unused-variable', '-Wno-unused-but-set-variable']
 
         if configuration['safe-math']:
@@ -400,6 +400,12 @@ def __init__(self, *args, **kwargs):
             # from `=512`
             self.cflags.append('-mprefer-vector-width=512')
 
+        if platform in [POWER8, POWER9]:
+           # -march isn't supported on power architectures, is -mtune needed?
+           self.cflags = ['-mcpu=native']  + self.cflags
+        else:
+           self.cflags = ['-march=native'] + self.cflags
+
         language = kwargs.pop('language', configuration['language'])
         try:
             if self.version >= Version("4.9.0"):