[Gold, aarch64] Implement some AArch64 relocs.

[Gold, aarch64] Implement some AArch64 relocs.

This CL implemented the following relocs for AArch64 target.
  - R_AARCH64_MOVW_UABS_G*
  - R_AARCH64_MOVW_SABS_G* relocations

gold/ChangeLog

2016-07-26 Igor Kudrin  <ikudrin@accesssoftek.com>

    * aarch64-reloc-property.cc (Rvalue_bit_select_impl): New class.
    (rvalue_bit_select): Use Rvalue_bit_select_impl.
    * aarch64-reloc.def (MOVW_UABS_G0, MOVW_UABS_G0_NC,
    MOVW_UABS_G1,
    MOVW_UABS_G1_NC, MOVW_UABS_G2, MOVW_UABS_G2_NC, MOVW_UABS_G3,
    MOVW_SABS_G0, MOVW_SABS_G1, MOVW_SABS_G2): New relocations.
    * aarch64.cc (Target_aarch64::Scan::local): Add cases for new
    MOVW_UABS_* and MOVW_SABS_* relocations.
    (Target_aarch64::Scan::global): Likewise.
    (Target_aarch64::Relocate::relocate): Add cases and handlings
    for new MOVW_UABS_* and MOVW_SABS_* relocations.
    * testsuite/Makefile.am (aarch64_relocs): New test.
    * testsuite/Makefile.in: Regenerate.
    * testsuite/aarch64_globals.s: New test source file.
    * testsuite/aarch64_relocs.s: Likewise.
    * testsuite/aarch64_relocs.sh: New test script.