Get SAMD51 CXX flags from script

platformio.ini

 [env:SAMD51_grandcentral_m4]
 platform       = atmelsam
 board          = adafruit_grandcentral_m4
-build_flags    = ${common.build_flags} -std=gnu++17 -Wno-register
+build_flags    = ${common.build_flags} -std=gnu++17
+extra_scripts  = ${common.extra_scripts}
 build_unflags  = -std=gnu++11
 src_filter     = ${common.default_src_filter} +<src/HAL/SAMD51>
 lib_deps       = ${common.lib_deps}