exec: Use const alias for TARGET_PAGE_BITS_VARY

exec: Use const alias for TARGET_PAGE_BITS_VARY

Using a variable that is declared "const" for this tells the
compiler that it may read the value once and assume that it
does not change across function calls.

For target_page_size, this means we have only one assert per
function, and one read of the variable.

This reduces the size of qemu-system-aarch64 by 8k.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>