target/i386: floatx80: avoid compound literals in static initializers

Quoting ISO C99 6.7.8p4, "All the expressions in an initializer for an
object that has static storage duration shall be constant expressions or
string literals".

The compound literal produced by the make_floatx80() macro is not such a
constant expression, per 6.6p7-9. (An implementation may accept it,
according to 6.6p10, but is not required to.)

Therefore using "floatx80_zero" and make_floatx80() for initializing
"f2xm1_table" and "fpatan_table" is not portable. And gcc-4.8 in RHEL-7.6
actually chokes on them:

> target/i386/fpu_helper.c:871:5: error: initializer element is not constant
>      { make_floatx80(0xbfff, 0x8000000000000000ULL),
>      ^

We've had the make_floatx80_init() macro for this purpose since commit
3bf7e40 ("softfloat: fix for C99", 2012-03-17), so let's use that
macro again.

Fixes: eca3064 ("target/i386: reimplement f2xm1 using floatx80 operations")
Fixes: ff57bb7 ("target/i386: reimplement fpatan using floatx80 operations")
Signed-off-by: Laszlo Ersek <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Cc: Aurelien Jarno <[email protected]>
Cc: Eduardo Habkost <[email protected]>
Cc: Joseph Myers <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Peter Maydell <[email protected]>
Link: https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg06566.html
Link: https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg04714.html
Message-Id: <[email protected]>
Message-Id: <[email protected]>

Author: Laszlo Ersek

include/fpu/softfloat.h

@@ -822,6 +822,7 @@ static inline bool floatx80_invalid_encoding(floatx80 a)
 }
 
 #define floatx80_zero make_floatx80(0x0000, 0x0000000000000000LL)
+#define floatx80_zero_init make_floatx80_init(0x0000, 0x0000000000000000LL)
 #define floatx80_one make_floatx80(0x3fff, 0x8000000000000000LL)
 #define floatx80_ln2 make_floatx80(0x3ffe, 0xb17217f7d1cf79acLL)
 #define floatx80_pi make_floatx80(0x4000, 0xc90fdaa22168c235LL)