devel/plan9port: fix build on GCC architectures

Use newer GCC:
cc1: error: unrecognized command line option "-Wno-misleading-indentation"
cc1: error: unrecognized command line option "-Wno-stringop-truncation"
cc1: error: unrecognized command line option "-Wno-stringop-overflow"
cc1: error: unrecognized command line option "-Wno-format-truncation"

MFH: 2020Q2 (fix build blanket)