base,arch-sparc: Overhaul the small fenv wrapper in base.

This was written in C, broke the style guide, used macros, used an
obsolete m5_ prefix, and used extern "C" in an odd way.

Change-Id: If38f9a1bca6fd4a4f7f533ddb1e81d6207bc9c44
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40878
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/SConscript

@@ -43,7 +43,7 @@ Executable('cprintftime', 'cprintftime.cc', 'cprintf.cc')
 Source('debug.cc')
 GTest('debug.test', 'debug.test.cc', 'debug.cc')
 if env['USE_FENV']:
-    Source('fenv.c')
+    Source('fenv.cc')
 else:
     warning("No IEEE FP rounding mode control.\n"
             "FP results may deviate slightly from other platforms.")