[libc] Enable fmaf and fma on x86_64.

They require clang-11 or above for building and hence had to be disabled
as the bots did not have clang-11 or higher. Bots have now been upgraded
so we can enable these functions now.

libc/config/linux/x86_64/entrypoints.txt

@@ -78,12 +78,8 @@ set(TARGET_LIBM_ENTRYPOINTS
     libc.src.math.floor
     libc.src.math.floorf
     libc.src.math.floorl
-    # Temporarily disable fma as the clang version
-    # on the bots cannot compile them. Note however
-    # that the version of gcc on the bots is able to
-    # compile them.
-    # libc.src.math.fma
-    # libc.src.math.fmaf
+    libc.src.math.fma
+    libc.src.math.fmaf
     libc.src.math.fmin
     libc.src.math.fminf
     libc.src.math.fminl