There should be something after the hyphen. Blame it on the cheap filter I suggested: ‘grep mtune=’. I think it may have found the target string mtune= near the end of a line, so whatever followed the hyphen after cortex- wrapped to the next line, fooling the simple-minded grep filter. Try:
gcc -mtune=native -Q --help=target
without piping to grep. You’ll get a much larger output. Look for a line (something like) The following options are target specific: then a long list of equivalencies, alphabetically sorted. Without filtering, you should find what is after the hyphen. It may be something like cortex-A or cortex-M4 or cortex-M or something else.
Compiling gmic on/for an Android is a bit of an outlier. Wear a helmet; you’ll probably hit other brick walls down this path. But, with patience, you’ll work through them.
EDIT:
Could also omit mtune= and march= entirely and let the compiler use its defaults. May not be optimal, but — 'build first; optimize later.'