mail-mta/exim-4.97.1-r6 merge fails on aarch64-musl-clang

[necktwi]

Core3566 ~ # emerge -pqv =mail-mta/exim-4.97.1-r5::gentoo | wgetpaste -s 0x0
Your paste can be seen here: http://0x0.st/Xis7.txt
Core3566 ~ # emerge --info '=mail-mta/exim-4.97.1-r5::gentoo' | wgetpaste -s 0x0
!!! No gcc found. You probably need to 'source /etc/profile'
!!! to update the environment of this terminal and possibly
!!! other terminals also.
Your paste can be seen here: http://0x0.st/Xish.bin
Core3566 ~ # wgetpaste -s 0x0 /var/tmp/portage/mail-mta/exim-4.97.1-r5/temp/build.log
Your paste can be seen here: http://0x0.st/XisD.log
Core3566 ~ # wgetpaste -s 0x0 /var/tmp/portage/mail-mta/exim-4.97.1-r5/temp/environment
Your paste can be seen here: http://0x0.st/Xisk.txt

It failed after adding `spf` use flag