libsigsegv: make the architecture dependencies more robust

libsigsegv: make the architecture dependencies more robust

While musl and glibc provide the ucontext structure definition on all
architectures they support, it is not the case of uClibc. We have been
excluding more and more architectures over time, but this isn't really
nice as we continuously need to add more. Latest example: the fact that
ucontext on ARM is only available on ARM platforms that support the ARM
instruction set (i.e not on the Thumb-2 only Cortex-M platforms).

Therefore, we introduce a BR2_PACKAGE_LIBSIGSEGV_ARCH_SUPPORTS variable,
which:

 - is set to 'y' for musl and glibc

 - is set to 'y' for uClibc for the only architectures that are known to
   provide ucontext (those are the ones that select ARCH_HAS_UCONTEXT in
   uClibc.

Note that we remove the comment, as it would become a way too
complicated comment, and we usually don't add comment for such
complicated situations (see libunwind for a similar example).

Fixes:

  http://autobuild.buildroot.net/results/b1e40b3ec64cd98b535d83e89e5780c46680e095/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>