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

@@ -397,7 +397,7 @@ def __init__(self, *args, **kwargs):
             self.cflags.append('-mprefer-vector-width=512')
 
         if platform in [POWER8, POWER9]:
-           # -march isn't supported on power architectures
+           # -march isn't supported on power architectures, is -mtune needed?
            self.cflags = ['-mcpu=native']  + self.cflags
         else:
            self.cflags = ['-march=native'] + self.cflags