Fix usage of signbit() in SRFI 144

C standard defines signbit() as a macro returning "non-zero value" for
negative arguments (see 7.12.3.6 of C11 standard). SRFI 144's flsign-bit
is defined to return exactly 1.

Make sure to convert the result of signbit() call into "boolean int"
which is either 0 or 1.

This is not a theoretical issue. This causes SRFI 144 test suite to fail
on many architectures that are not x86_64.

GCC on x86_64 compiles signbit() as

        movmskpd %xmm0, %eax
        andl     $1, %eax

which indeed returns either 0 or 1. movmskpd extracts 2-bit sign mask
from the FP value in src register and stores that in low-order bits of
the dst register. Then the unneded extra bit is masked out, leaving only
the lowest bit set or unset.

However, other architectures don't have such conveniences and go with
more direct approach. For example, GCC on ARMv7 produces this:

        sub     sp, sp, #8
        vstr.64 d0, [sp]
        ldr     r0, [sp, #4]
        and     r0, r0, #0x80000000
        add     sp, sp, #8
        bx      lr

which effectively returns either 0 or -1. Generated code masks out
everything but the sign bit and returns the result as is. The value
0x80000000 is the representation of -1.

Even on i386 signbit() is compiled as

        fldl    4(%esp)
        fxam
        fnstsw  %ax
        fstp    %st(0)
        andl    $512, %eax
        ret

which effectively returns either 0 or 512: fxam sets C1 bit FPU status
word to the sign of FP value, then the status word is extracted, the
"sign bit" is masked out, and left as is.

lib/srfi/144/math.stub

@@ -102,7 +102,8 @@
 
 (define-c double frexp (double (result int)))
 
-(define-c int (sign-bit "signbit") (double))
+(c-declare "#define sign_bit(v) (!!signbit(v))")
+(define-c int sign-bit (double))
 
 (define-c boolean (flfinite? "isfinite") (double))
 (define-c boolean (flinfinite? "isinf") (double))