configure: unify case statements for CPU canonicalization

configure: unify case statements for CPU canonicalization

The CPU model has to be canonicalized to what Meson wants in the cross
file, to what Linux uses for its asm-$ARCH directories, and to what
QEMU uses for its user-mode emulation host/$ARCH directories.  Do
all three in a single case statement, and check that the Linux and
QEMU directories actually exist.

At a small cost in repeated lines, this ensures that there are no hidden
ordering requirements between the case statements.  In particular, commit
89e5b7935e9 ("configure: Fix linux-user host detection for riscv64",
2023-08-06) broke ppc64le because it assigned host_arch based on a
non-canonicalized version of $cpu.

Reported-by: Joel Stanley <joel@jms.id.au>
Fixes: 89e5b7935e9 ("configure: Fix linux-user host detection for riscv64", 2023-08-06)
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Message-ID: <20230808120303.585509-4-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>