target/m68k: Switch fpu_rom from make_floatx80() to make_floatx80_init()

GCC 4.7.2 on SunOS reports that the values assigned to array members are not
real constants:

target/m68k/fpu_helper.c:32:5: error: initializer element is not constant
target/m68k/fpu_helper.c:32:5: error: (near initialization for 'fpu_rom[0]')
rules.mak:66: recipe for target 'target/m68k/fpu_helper.o' failed

Convert the array to make_floatx80_init() to fix it.
Replace floatx80_pi-like constants with make_floatx80_init() as they are
defined as make_floatx80().

This fixes build on SmartOS (Joyent).

Signed-off-by: Kamil Rytarowski <n54@gmx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170904212306.3020-1-n54@gmx.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
