I just wanted to compile SerenityOS on a whim, but got stopped immediately by my CXXFLAGS="-march=native". But the error is weird:

[53/3993] Building CXX object Kernel/CMakeFiles/kernel_heap.dir/Heap/SlabAllocator.cpp.o                                                                                                      
FAILED: Kernel/CMakeFiles/kernel_heap.dir/Heap/SlabAllocator.cpp.o                                                                                                                            
/home/ich/devel/serenity/Toolchain/Local/i686//bin/i686-pc-serenity-g++ -DENABLE_COMPILETIME_FORMAT_CHECK -DKERNEL -DSANITIZE_PTRS -D__serenity__ -I/home/ich/devel/serenity/Userland/Librarie
s -I/home/ich/devel/serenity/. -I/home/ich/devel/serenity/Build/i686 -I/home/ich/devel/serenity/Userland/Libraries/LibC -I/home/ich/devel/serenity/Userland/Libraries/LibCrypt -I/home/ich/dev
el/serenity/Userland/Libraries/LibM -I/home/ich/devel/serenity/Userland/Libraries/LibPthread -I/home/ich/devel/serenity/Userland/Libraries/LibSystem -I/home/ich/devel/serenity/Userland/Servi
ces -I/home/ich/devel/serenity/Userland -I/home/ich/devel/serenity/Build/i686/Userland/Services -I/home/ich/devel/serenity/Build/i686/Userland/Libraries -I/home/ich/devel/serenity/Build/i686
/Userland -I/home/ich/devel/serenity/Toolchain/Local/i686/i686-pc-serenity/include/c++/11.2.0 -I/home/ich/devel/serenity/Toolchain/Local/i686/i686-pc-serenity/include/c++/11.2.0/i686-pc-sere
nity -march=native -pipe -frecord-gcc-switches -pie -fpic -Wno-unknown-warning-option -Wvla -Wnull-dereference -fno-rtti -ffreestanding -fbuiltin -mno-80387 -mno-mmx -mno-sse -mno-sse2 -fno-
asynchronous-unwind-tables -fstack-protector-strong -fno-exceptions -nodefaultlibs -nostdlib -nostdinc -nostdinc++ -fzero-call-used-regs=used-gpr -faligned-new=4 -static-pie -fsanitize=undef
ined -Wno-literal-suffix -Wformat=2 -fdiagnostics-color=always -Werror -Wall -Wextra -Wno-address-of-packed-member -Wcast-align -Wcast-qual -Wno-deprecated-copy -Wduplicated-cond -Wdouble-pr
omotion -Wno-expansion-to-defined -Wimplicit-fallthrough -Wlogical-op -Wmisleading-indentation -Wmissing-declarations -Wno-nonnull-compare -Wnon-virtual-dtor -Wno-unknown-warning-option -Wun
def -Wunused -Wwrite-strings -Wno-maybe-uninitialized -ffile-prefix-map=/home/ich/devel/serenity=. -fno-exceptions -fstack-protector-strong -g1 -fstack-clash-protection -Os -std=c++20 -MD -M
T Kernel/CMakeFiles/kernel_heap.dir/Heap/SlabAllocator.cpp.o -MF Kernel/CMakeFiles/kernel_heap.dir/Heap/SlabAllocator.cpp.o.d -o Kernel/CMakeFiles/kernel_heap.dir/Heap/SlabAllocator.cpp.o -c
 /home/ich/devel/serenity/Kernel/Heap/SlabAllocator.cpp                                                                                                                                       
cc1plus: error: bad value ('native') for '-march=' switch                                                                                                                                     
cc1plus: note: valid arguments to '-march=' switch are: i386 i486 i586 pentium lakemont pentium-mmx winchip-c6 winchip2 c3 samuel-2 c3-2 nehemiah c7 esther i686 pentiumpro pentium2 pentium3 
pentium3m pentium-m pentium4 pentium4m prescott nocona core2 nehalem corei7 westmere sandybridge corei7-avx ivybridge core-avx-i haswell core-avx2 broadwell skylake skylake-avx512 cannonlake
 icelake-client rocketlake icelake-server cascadelake tigerlake cooperlake sapphirerapids alderlake bonnell atom silvermont slm goldmont goldmont-plus tremont knl knm geode k6 k6-2 k6-3 athl
on athlon-tbird athlon-4 athlon-xp athlon-mp x86-64 x86-64-v2 x86-64-v3 x86-64-v4 eden-x2 nano nano-1000 nano-2000 nano-3000 nano-x2 eden-x4 nano-x4 k8 k8-sse3 opteron opteron-sse3 athlon64 
athlon64-sse3 athlon-fx amdfam10 barcelona bdver1 bdver2 bdver3 bdver4 znver1 znver2 znver3 btver1 btver2 native

So it claims that -march=native should be supported but fails. Where is that compiler binary even coming from and why doesn't it want -march=native?