math: provide some more floating point symbols

Applications may require symbols like __isinf, __isnanl, etc. The standard
math.h header only provides macros. We currently have an implementation for
__isnan, but not for other important symbols that are being requested by the
application.

C++ have implementations of it, so there is no need to go for craft assemble.
I trully have no idea if our asm inline was more or less efficient than the C++
version, but I can't honestly imagine a program calling isinf in a loop 1
million times per sec.

For the record, it is an obvious possibility that the C++ implementation itself
could use those symbols, but I verified they don't. At least for x86. Example
code:

0000000000000040 <__isinfl>:
  40:   f2 0f 10 0d 00 00 00    movsd  0x0(%rip),%xmm1        # 48 <__isinfl+0x8>
  47:   00
  48:   31 c0                   xor    %eax,%eax
  4a:   55                      push   %rbp
  4b:   66 0f 54 c1             andpd  %xmm1,%xmm0
  4f:   48 89 e5                mov    %rsp,%rbp
  52:   5d                      pop    %rbp
  53:   66 0f 2e 05 00 00 00    ucomisd 0x0(%rip),%xmm0        # 5b <__isinfl+0x1b>
  5a:   00
  5b:   0f 97 c0                seta   %al
  5e:   c3                      retq

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
CC: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>