superenv: refurbish --fast-math for :clang

"--fast-math" caused build failure with clang for
homebrew/science/delly-0.7.2

Apparently, clang only likes this option when it's referred to as
"-ffast-math"

Closes Homebrew/homebrew#50507.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

Library/ENV/4.3/cc

@@ -158,6 +158,9 @@ class Cmd
       "-fuse-linker-plugin"
       # clang doesn't support these flags
       args << arg unless tool =~ /^clang/
+    when "--fast-math"
+      arg = "-ffast-math" if tool =~ /^clang/
+      args << arg
     when "-Wno-deprecated-register"
       # older gccs don't support these flags
       args << arg unless tool =~ /^g..-4.[02]/