RFR: 8200168: Remove DONT_USE_REGISTER_DEFINES on Sparc

[Per Liden]

Hi,

Please review this patch to remove register macros on Sparc, as 
discussed here: 
http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2018-March/028541.html

Bug: https://bugs.openjdk.java.net/browse/JDK-8200168
Webrev: http://cr.openjdk.java.net/~pliden/8200168/webrev.0

Testing: Passed hs-tier{1,2}

/Per