Add descriptions for WITH_ASAN and WITH_UBSAN

[emaste]

Summary

Fixes: R10:7bc797e3f380 ("Add build system support for ASAN+UBSAN instrumentation")

[jrtc27]

Also requires some of the sanitiser run-time libraries from lib/libclang_rt

[emaste]

They get built automatically when compiling with Clang - there's no option that needs to be turned on for them. My intent with this statement was to report that the option doesn't work if using GCC (via external toolchain). Any suggestion to make that more clear?

The generated src.conf.5 has:

+.It Va WITH_ASAN
+Build the base system with Address Sanitizer (ASan) to detect
+memory corruption bugs such as buffer overflows or use-after-free.
+Requires Clang as base system compiler.
+When set, it enforces these options:
+.Pp
+.Bl -item -compact
+.It
+.Va WITH_LLVM_BINUTILS
+.It
+.Va WITH_LLVM_CXXFILT
+.El

[jrtc27]

In D32773#739600, @emaste wrote:

They get built automatically when compiling with Clang - there's no option that needs to be turned on for them

Sure, but they're only built for a limited number of architectures

[emaste]

Sure, but they're only built for a limited number of architectures.

Ah yes, good point. Maybe "Requires that Clang be used as the base system compiler and that the runtime support library is available."

[emaste]

• Mention runtimes based on @jrtc27's comment
• Add generated src.conf.5 for reference