docs: Correct subject prefix and update LLVM info

bpf_devel_QA.rst:152 The subject prefix information is not accurate, it
should be 'PATCH bpf-next v2'

Also update LLVM version info and add information about
‘-DLLVM_TARGETS_TO_BUILD’ to prompt the developer to build the desired
target.

Signed-off-by: Jianlin Lv <Jianlin.Lv@arm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200821052817.46887-1-Jianlin.Lv@arm.com

Documentation/bpf/bpf_devel_QA.rst

@@ -149,7 +149,7 @@ In case the patch or patch series has to be reworked and sent out
 again in a second or later revision, it is also required to add a
 version number (``v2``, ``v3``, ...) into the subject prefix::
 
-  git format-patch --subject-prefix='PATCH net-next v2' start..finish
+  git format-patch --subject-prefix='PATCH bpf-next v2' start..finish
 
 When changes have been requested to the patch series, always send the
 whole patch series again with the feedback incorporated (never send
@@ -479,12 +479,13 @@ LLVM's static compiler lists the supported targets through
 
      $ llc --version
      LLVM (http://llvm.org/):
-       LLVM version 6.0.0svn
+       LLVM version 10.0.0
        Optimized build.
        Default target: x86_64-unknown-linux-gnu
        Host CPU: skylake
 
        Registered Targets:
+         aarch64    - AArch64 (little endian)
          bpf        - BPF (host endian)
          bpfeb      - BPF (big endian)
          bpfel      - BPF (little endian)
@@ -517,6 +518,10 @@ from the git repositories::
 The built binaries can then be found in the build/bin/ directory, where
 you can point the PATH variable to.
 
+Set ``-DLLVM_TARGETS_TO_BUILD`` equal to the target you wish to build, you
+will find a full list of targets within the llvm-project/llvm/lib/Target
+directory.
+
 Q: Reporting LLVM BPF issues
 ----------------------------
 Q: Should I notify BPF kernel maintainers about issues in LLVM's BPF code