std.math.frexp wrong on ARM

[Johannes Pfau]

The frexp test fails on ARM. I think the mask in line 1491 is
wrong:

https://github.com/D-Programming-Language/phobos/blob/master/std/math.d#L1491
For doubles, the 63 bit is sign, 62-52 are exponent and 51-0 are
mantissa.
The mask manipulates the bits 63-48 (ushort, 16bit)
0x8000 is 0b1000000000000000
so it preserves the sign, but not the 4 bits of the mantissa? I think
it should be 
0b1000000000001111 (0x800F)?
This also fixes the test case on ARM.

But I don't know much about this stuff, so I wonder if this is correct?

(BTW: Shouldn't we use the binary representation for bitmasks instead?)