Fix const doubles on macOS

[cadmic]

Currently the macOS version miscompiles basically anything involving double constants. On 32-bit machines (e.g. Linux builds), for a struct rtx_def *r that represents a const double the value is split into two parts, accessed as

#define CONST_DOUBLE_LOW(r) XWINT (r, 2)  // r->fld[2].rtwint
#define CONST_DOUBLE_HIGH(r) XWINT (r, 3) // r->fld[3].rtwint

and the value is usually copied out like

union real_extract u;
bcopy ((char *) &CONST_DOUBLE_LOW (r), (char *) &u, sizeof (u));

which assumes that CONST_DOUBLE_LOW and CONST_DOUBLE_HIGH are 4 bytes apart in memory. On macOS we compile as 64-bit though, meaning this bcopy will ignore CONST_DOUBLE_HIGH.

Currently on macOS HOST_BITS_PER_LONG is 32 and HOST_WIDE_INT is int which is kind of a lie. Technically we could fix const doubles by changing HOST_BITS_PER_LONG to 64 (then the code would put entire double values in CONST_DOUBLE_LOW instead of splitting it into two parts) but I'm concerned that this would break matching, since other code uses CONST_DOUBLE_LOW/CONST_DOUBLE_HIGH directly for hashing and such.

Updating all the backends was probably not necessary but it's nice to be consistent I guess.

[AngheloAlf]

I agree this solution looks better than papermario's one.

btw, using bcopy and bcmp just for 8 bytes sounds pretty overkill and error prone, I wonder why they went with it.

Also checked drmario does still match 👍