RFR (trivial): 8219519: Remove linux_sparc.ad and linux_aarch64.ad

[Jie Fu]

Hi all,

The following two source files are useless for the generation of C2 and 
should be removed.
  1) ./src/hotspot/os_cpu/linux_sparc/linux_sparc.ad
  2) ./src/hotspot/os_cpu/linux_aarch64/linux_aarch64.ad

Bug:    https://bugs.openjdk.java.net/browse/JDK-8219519
Webrev: http://cr.openjdk.java.net/~jiefu/8219519/webrev.00/

Could you please review it?
Thanks a lot.

Best regards,
Jie