'./configure --cpu=$(uname -m)' (essentially) breaks the case of riscv,
where the configure script expects '--cpu=riscv', but 'uname -m' gives
'riscv{32,64}'. This eventually leads to user-mode emulation being
disabled.
Since the configure script can auto-detect the parameter based on
compilers, remove this flag.
Originally added in #147692, but apparently it's not needed and @NickCao
couldn't remember why it was added. See also mailing list discussion at
https://lore.kernel.org/qemu-devel/20230204112502.2558739-1-mjt@msgid.tls.msk.ru/T/