[15] RFR (S): 8238683: C2: Remove Use24BitFP and Use24BitFPMode develop flags

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8238683/webrev.00/
http://jbs.oracle.com/browse/JDK-8238683

Remove Use24BitFP and Use24BitFPMode develop flags.

The functionality is used only on x86-32, so I wrapped relevant code in 
#ifdefs.

Testing: linux-x86 fastdebug build, hs-precheckin-comp, hs-tier1, hs-tier2

Thanks!

Best regards,
Vladimir Ivanov