mongrel2: allow using uClibc but only on certain architectures

mongrel2 uses the {get,make,swap}context functions:

 - With glibc, no problem, they are available on all supported
   architectures

 - With uClibc, they are available only on a subset of the
   architectures. Until now, only BR2_UCLIBC_VERSION_SNAPSHOT
   configurations were allowed to select mongrel2, but we are going to
   get rid of the uClibc snapshot version, and uClibc-ng is as capable
   as the uClibc snapshot. However, only certain architectures have
   the *context() functions.

 - With musl, there is no *context() support.

Since this dependency is quite complicated, we introduce a
BR2_PACKAGE_MONGREL2_LIBC_SUPPORTS hidden boolean to encode which C
libraries are supported.

Also, listing the supported uClibc architectures would be too long in
the comment, so we simply indicate that the package needs uClibc or
glibc.

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