aarch64: disable feature b16b16

Feature b16b16 is currently incomplete and requires re-work.

Disable the command line option for b16b16, and mark the associated
tests as XFAIL.

gas/config/tc-aarch64.c

diff --git a/gas/config/tc-aarch64.c b/gas/config/tc-aarch64.c
index 8690b8f..ed5ab76 100644
--- a/gas/config/tc-aarch64.c
+++ b/gas/config/tc-aarch64.c
@@ -10705,7 +10705,9 @@
   {"rasv2",		AARCH64_FEATURE (RASv2), AARCH64_FEATURE (RAS)},
   {"ite",		AARCH64_FEATURE (ITE), AARCH64_NO_FEATURES},
   {"d128",		AARCH64_FEATURE (D128), D128_FEATURE_DEPS},
-  {"b16b16",		AARCH64_FEATURE (B16B16), AARCH64_FEATURE (SVE2)},
+  // Feature b16b16 is currently incomplete.
+  // TODO: finish implementation and enable relevant flags.
+  //{"b16b16",		AARCH64_FEATURE (B16B16), AARCH64_FEATURE (SVE2)},
   {"sme2p1",		AARCH64_FEATURE (SME2p1), AARCH64_FEATURE (SME2)},
   {"sve2p1",		AARCH64_FEATURE (SVE2p1), AARCH64_FEATURE (SVE2)},
   {"rcpc3",		AARCH64_FEATURE (RCPC3), AARCH64_FEATURE (RCPC2)},

gas/doc/c-aarch64.texi

diff --git a/gas/doc/c-aarch64.texi b/gas/doc/c-aarch64.texi
index 2126304..8e46038 100644
--- a/gas/doc/c-aarch64.texi
+++ b/gas/doc/c-aarch64.texi
@@ -161,8 +161,8 @@
 @headitem Extension @tab Depends upon @tab Description
 @item @code{aes} @tab @code{simd}
  @tab Enable the AES and PMULL cryptographic extensions.
-@item @code{b16b16} @tab @code{sve2}
- @tab Enable BFloat16 to BFloat16 arithmetic for SVE2 and SME2.
+@c @item @code{b16b16} @tab @code{sve2}
+@c  @tab Enable BFloat16 to BFloat16 arithmetic for SVE2 and SME2.
 @item @code{bf16} @tab @code{fp}
  @tab Enable BFloat16 extension.
 @item @code{brbe} @tab

gas/testsuite/gas/aarch64/bfloat16-1.d

diff --git a/gas/testsuite/gas/aarch64/bfloat16-1.d b/gas/testsuite/gas/aarch64/bfloat16-1.d
index 51f7e6c..68a47f5 100644
--- a/gas/testsuite/gas/aarch64/bfloat16-1.d
+++ b/gas/testsuite/gas/aarch64/bfloat16-1.d
@@ -1,6 +1,7 @@
 #name: Test of SVE2.1 and SME2.1 non-widening BFloat16 instructions.
 #as: -march=armv9.4-a+b16b16
 #objdump: -dr
+#xfail: *-*-*
 
 [^:]+:     file format .*
 

gas/testsuite/gas/aarch64/bfloat16-2-invalid.d

diff --git a/gas/testsuite/gas/aarch64/bfloat16-2-invalid.d b/gas/testsuite/gas/aarch64/bfloat16-2-invalid.d
index 1cd2745..1e1b701 100644
--- a/gas/testsuite/gas/aarch64/bfloat16-2-invalid.d
+++ b/gas/testsuite/gas/aarch64/bfloat16-2-invalid.d
@@ -2,3 +2,4 @@
 #as: -march=armv9.4-a+b16b16
 #source: bfloat16-2-invalid.s
 #error_output: bfloat16-2-invalid.l
+#xfail: *-*-*