hyper ~ # grep FLAGS /etc/portage/make.conf
COMMON_FLAGS="-march=znver1 -O2 -pipe"
CFLAGS="${COMMON_FLAGS}"
CXXFLAGS="${COMMON_FLAGS}"
FCFLAGS="${COMMON_FLAGS}"
FFLAGS="${COMMON_FLAGS}"
CPU_FLAGS_X86="aes avx avx2 f16c fma3 mmx mmxext pclmul popcnt sha sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3"
RUSTFLAGS="-C target-cpu=znver1"

hyper ~ # cat /etc/portage/package.env 
sys-devel/gcc gcc.conf

hyper ~ # cat /etc/portage/env/gcc.conf 
COMMON_FLAGS="-O2 -pipe"
CFLAGS="${COMMON_FLAGS}"
CXXFLAGS="${COMMON_FLAGS}"
FCFLAGS="${COMMON_FLAGS}"
FFLAGS="${COMMON_FLAGS}"