tests/fp/berkeley-testfloat-3: Ignore ignored #pragma directives

Since we already use -Wno-unknown-pragmas, we can also use
-Wno-ignored-pragmas. This silences hundred of warnings using
clang 13 on macOS Monterey:

  [409/771] Compiling C object tests/fp/libtestfloat.a.p/berkeley-testfloat-3_source_test_az_f128_rx.c.o
  ../tests/fp/berkeley-testfloat-3/source/test_az_f128_rx.c:49:14: warning: '#pragma FENV_ACCESS' is not supported on this target - ignored [-Wignored-pragmas]
  #pragma STDC FENV_ACCESS ON
               ^
  1 warning generated.

Having:

  $ cc -v
  Apple clang version 13.0.0 (clang-1300.0.29.30)

Reported-by: Roman Bolshakov <roman@roolebo.dev>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Tested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

tests/fp/meson.build

@@ -37,6 +37,11 @@ tfcflags = [
   '-Wno-error',
 ]
 
+if cc.get_id() == 'clang'
+  # Clang does not support '#pragma STDC FENV_ACCESS'
+  tfcflags += [ '-Wno-ignored-pragmas' ]
+endif
+
 tfgencases = [
   tfdir / 'genCases_ui32.c',
   tfdir / 'genCases_ui64.c',