Most of emulated CPU have instructions aligned on 16 or 32 bits, while
on others GCC tries to align the target jump location. This means that
1/2 or 3/4 of tb_phys_hash entries are never used.

Update the hash function tb_phys_hash_func() to ignore the two lowest
bits of the address. This brings a 6% speed-up when booting a MIPS
image.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>