[PATCH] Guard declarations of 'sve_*_from_*' macros on Aarch64 (and unbreak build)

[Sergio Durigan Junior <sergiodj@redhat.com>]

Commit 122394f1476b1c925a281b15399119500c8231c1 ("Function for reading
the Aarch64 SVE vector length") has added macros to manipulate SVE
vector sizes based on Linux kernel sources, but did not guard them
with #ifndef's, which breaks the build when the system headers already
have these macros:

    CXX    aarch64-linux-nat.o
  In file included from ../../gdb/aarch64-tdep.h:25,
                   from ../../gdb/aarch64-linux-nat.c:30:
  ../../gdb/arch/aarch64.h:79: error: "sve_vq_from_vl" redefined [-Werror]
   #define sve_vq_from_vl(vl) ((vl) / 0x10)

  In file included from /usr/include/bits/sigcontext.h:30,
                   from /usr/include/signal.h:291,
                   from build-gnulib/import/signal.h:52,
                   from ../../gdb/linux-nat.h:23,
                   from ../../gdb/aarch64-linux-nat.c:26:
  /usr/include/asm/sigcontext.h:154: note: this is the location of the previous definition
   #define sve_vq_from_vl(vl) ((vl) / SVE_VQ_BYTES)

  In file included from ../../gdb/aarch64-tdep.h:25,
                   from ../../gdb/aarch64-linux-nat.c:30:
  ../../gdb/arch/aarch64.h:80: error: "sve_vl_from_vq" redefined [-Werror]
   #define sve_vl_from_vq(vq) ((vq) * 0x10)

  In file included from /usr/include/bits/sigcontext.h:30,
                   from /usr/include/signal.h:291,
                   from build-gnulib/import/signal.h:52,
                   from ../../gdb/linux-nat.h:23,
                   from ../../gdb/aarch64-linux-nat.c:26:
  /usr/include/asm/sigcontext.h:155: note: this is the location of the previous definition
   #define sve_vl_from_vq(vq) ((vq) * SVE_VQ_BYTES)

In order to fix this breakage, this commit guards the declaration of
the macros using #ifndef's.  It is important to mention that the
system headers use a value to multiply/divide the vector
length (SVE_VQ_BYTES, defined as 16 on the system I'm using) which is
different than the values being used by the macros defined in GDB.  I
wasn't sure how to consolidate that, so I chose to ignore this
discrepancy.

Tested by making sure GDB compiles fine again.  Since the system I'm
using doesn't have support for SVE, there's no way I can really test
these changes.

gdb/ChangeLog:
2018-06-05  Sergio Durigan Junior  <sergiodj@redhat.com>

	* arch/aarch64.h (sve_vg_from_vl): Guard with #ifndef.
	(sve_vl_from_vg): Likewise.
	(sve_vq_from_vl): Likewise.
	(sve_vl_from_vq): Likewise.
	(sve_vq_from_vg): Likewise.
	(sve_vg_from_vq): Likewise.
---
 gdb/arch/aarch64.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/gdb/arch/aarch64.h b/gdb/arch/aarch64.h
index 9040d8d4c8..c378a4b239 100644
--- a/gdb/arch/aarch64.h
+++ b/gdb/arch/aarch64.h
@@ -74,12 +74,24 @@ enum aarch64_regnum
    VG : Vector Gradient.
 	The number of 64bit chunks in an SVE Z register.  */
 
+#ifndef sve_vg_from_vl
 #define sve_vg_from_vl(vl)	((vl) / 8)
+#endif
+#ifndef sve_vl_from_vg
 #define sve_vl_from_vg(vg)	((vg) * 8)
+#endif
+#ifndef sve_vq_from_vl
 #define sve_vq_from_vl(vl)	((vl) / 0x10)
+#endif
+#ifndef sve_vl_from_vq
 #define sve_vl_from_vq(vq)	((vq) * 0x10)
+#endif
+#ifndef sve_vq_from_vg
 #define sve_vq_from_vg(vg)	(sve_vq_from_vl (sve_vl_from_vg (vg)))
+#endif
+#ifndef sve_vg_from_vq
 #define sve_vg_from_vq(vq)	(sve_vg_from_vl (sve_vl_from_vq (vq)))
+#endif
 
 
 /* Maximum supported VQ value.  Increase if required.  */
-- 
2.14.3