FreeBSD EXEC_PAGESIZE implemented by getpagesize()

On FreeBSD the EXEC_PAGESIZE constant does not exist. getpagesize() is a
system function to query the current page size. For the least
intrusiveness, EXEC_PAGESIZE is here defined to a call to getpagesize().
This is inefficient and could probably be cached.
FreeBSD lacks libdl, has libc instead

On FreeBSD, libc provides all the functionality that libdl does on other
OSes so there's no point in linking a non-existent library.
Add FreeBSD to platforms that use POSIX context

FreeBSD is POSIX:y enough to use the same sort of contexts like MacOS X
does, with the difference that the stack_t ss_sp field has type char*,
not void*.
Remove FreeBSD guards for Linux-only functionality

Some functionality does not have an immediately available mirror in FreeBSD:

* TCP_QUICKACK;
* what linux_topology.hpp uses to set thread affinity mask;
* constexpr replacement for EXEC_PAGESIZE, so no context_linux_x86.hpp.