arm64 Improve fpsr gdbsrv handling.

arm64 Improve fpsr gdbsrv handling.
let the compiler handle the ULong to UInt conversion rather than
play with addresses.

Tested manually GDB+vgdb that reading and setting fpsr works, using
code such as (provided by Julian, I cannot write a single line of
arm64 asm :)
 void set_fpsr ( uint32_t val ) {
     __asm__ __volatile__( "msr fpsr, %0" : : "r"(val) : "cc" );
  }

  uint32_t get_fpsr ( void ) {
     uint32_t res;
     __asm__ __volatile__( "mrs %0, fpsr" : "=r"(res) : : "cc" );
     return res;
  }

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14256