MIPS/user: Fix reset CPU state initialization

 This change updates the CPU reset sequence to use a common piece of code
that figures out CPU state flags, fixing the problem with MIPS_HFLAG_COP1X
not being set where applicable that causes floating-point MADD family
instructions (and other instructions from the MIPS IV FP subset) to trap.

 As compute_hflags is now shared between op_helper.c and translate.c, the
function is now moved to a common header.  There are no changes to this
function.

 The problem was seen with the 24Kf MIPS32r2 processor in user emulation.
The new approach prevents system and user emulation from diverging -- all
the hflags state is initialized in one place now.

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
