softfloat: Move floatN_log2 to softfloat-parts.c.inc

Rename to parts$N_log2.  Though this is partly a ruse, since I do not
believe the code will succeed for float128 without work.  Which is ok
for now, because we do not need this for more than float32 and float64.

Since berkeley-testfloat-3 doesn't support log2, compare float64_log2
vs the system log2.  Fix the errors for inputs near 1.0:

test: 3ff00000000000b0  +0x1.00000000000b0p+0
  sf: 3d2fa00000000000  +0x1.fa00000000000p-45
libm: 3d2fbd422b1bd36f  +0x1.fbd422b1bd36fp-45
Error in fraction: 32170028290927 ulp

test: 3feec24f6770b100  +0x1.ec24f6770b100p-1
  sf: bfad3740d13c9ec0  -0x1.d3740d13c9ec0p-5
libm: bfad3740d13c9e98  -0x1.d3740d13c9e98p-5
Error in fraction: 40 ulp

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tests/fp/meson.build

@@ -634,3 +634,14 @@ fpbench = executable(
   include_directories: [sfinc, include_directories(tfdir)],
   c_args: fpcflags,
 )
+
+fptestlog2 = executable(
+  'fp-test-log2',
+  ['fp-test-log2.c', '../../fpu/softfloat.c'],
+  link_with: [libsoftfloat],
+  dependencies: [qemuutil],
+  include_directories: [sfinc],
+  c_args: fpcflags,
+)
+test('fp-test-log2', fptestlog2,
+     suite: ['softfloat', 'softfloat-ops'])