selftests/bpf: Add SAN_CFLAGS param to selftests build to allow sanitizers

Add ability to specify extra compiler flags with SAN_CFLAGS for compilation of
all user-space C files.  This allows to build all of selftest programs with,
e.g., custom sanitizer flags, without requiring support for such sanitizers
from anyone compiling selftest/bpf.

As an example, to compile everything with AddressSanitizer, one would do:

  $ make clean && make SAN_CFLAGS="-fsanitize=address"

For AddressSanitizer to work, one needs appropriate libasan shared library
installed in the system, with version of libasan matching what GCC links
against. E.g., GCC8 needs libasan5, while GCC7 uses libasan4.

For CentOS 7, to build everything successfully one would need to:
  $ sudo yum install devtoolset-8-gcc devtoolset-libasan-devel
  $ scl enable devtoolset-8 bash # set up environment

For Arch Linux to run selftests, one would need to install gcc-libs package to
get libasan.so.5:
  $ sudo pacman -S gcc-libs

N.B. EXTRA_CFLAGS name wasn't used, because it's also used by libbpf's
Makefile and this causes few issues:
1. default "-g -Wall" flags are overriden;
2. compiling shared library with AddressSanitizer generates a bunch of symbols
   like: "_GLOBAL__sub_D_00099_0_btf_dump.c", "_GLOBAL__sub_D_00099_0_bpf.c",
   etc, which screws up versioned symbols check.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Julia Kartseva <hex@fb.com>
Link: https://lore.kernel.org/bpf/20200429012111.277390-3-andriin@fb.com