[PATCH][AArch64] Use builtins for fpcr/fpsr

[Joseph Myers]

I'm seeing:

In file included from ../include/fpu_control.h:2:0,
                 from test-fpucw.c:19:
../sysdeps/aarch64/fpu/fpu_control.h:24:5: error: "__GNUC_PREREQ" is not defined, evaluates to 0 [-Werror=undef]
 #if __GNUC_PREREQ (6,0)
     ^~~~~~~~~~~~~
../sysdeps/aarch64/fpu/fpu_control.h:24:19: error: missing binary operator before token "("
 #if __GNUC_PREREQ (6,0)
                   ^

fpu_control.h is missing an include of features.h, as needed to use 
__GNUC_PREREQ.

-- 
Joseph S. Myers
joseph@codesourcery.com