tests/tcg: Replace -mpower8-vector with -mcpu=power8

tests/tcg: Replace -mpower8-vector with -mcpu=power8

[1] deprecated -mpower8-vector, resulting in:

    powerpc64-linux-gnu-gcc: warning: switch '-mpower8-vector' is no longer supported
    qemu/tests/tcg/ppc64/vsx_f2i_nan.c:4:15: error: expected ';' before 'float'
        4 | typedef vector float vsx_float32_vec_t;
          |               ^~~~~~

Use -mcpu=power8 instead. In order to properly verify that this works,
one needs a big-endian (the minimum supported CPU for 64-bit
little-endian is power8 anyway) GCC configured with --enable-checking
(see GCC commit e154242724b0 ("[RS6000] Don't pass -many to the
assembler").

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109987

Cc: qemu-stable@nongnu.org
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>