tcg-sparc: Don't MAP_FIXED on top of the program

The address we pick in sparc64.ld is also 0x60000000, so doing a fixed map
on top of that is guaranteed to blow up.  Choosing 0x40000000 is exactly
right for the max of code_gen_buffer_size set below.

No need to ever use MAP_FIXED.  While getting our desired address helps
optimize the generated code, we won't fail if we don't get it.

Signed-off-by: Richard Henderson <rth@twiddle.net>