[set,get,make,swap]context from glibc for amd64 implementation and question for Norman

[Alexander Tormasov]

One more attempt to implement [set,get,make,swap]context inside libc scope, much smaller
- only asm files moved from previous attempt (overall around 400+- lines):
git apply --stat ../mc.patch
 repos/libports/lib/import/import-libc-context.mk   |    1
 repos/libports/lib/mk/libc.mk                      |    2
 repos/libports/lib/mk/spec/x86_64/libc-context.mk  |    9 +
 repos/libports/lib/symbols/libc                    |    2
 repos/libports/ports/libc.hash                     |    2
 repos/libports/recipes/api/libc_setjmp/content.mk  |    2
 repos/libports/src/lib/libc/dummies.cc             |   28 +-
 repos/libports/src/lib/libc/patches/mcontext.patch |  346 ++++++++++++++++++++
 8 files changed, 387 insertions(+), 5 deletions(-)

https://github.com/tor-m6/genode/commit/b5ed6a49b0b6c09b20161b1ea726dbb685ae5ba0

Also add test for them, see separate patch. Not yet choose where to put, potentially should be near makecontext() declaration inside libc include/libc/ucontext.h.
extern "C" void *alloc_secondary_stack(char const *name, size_t stack_size);


Josef, please, take a look for this patch and for anon_mmap! 

My understanding of limitations:
- x86_64 only (easy to extend to others like arm64)
- ignore set/restore of signals (need kernel support?)
- replace/add old code from libc references to interpose table/etc; not 100% sure that all related aux functions like __getcontextx will work as FreeBSD intended…
- require alloc_secondary_stack for correct operations in makecontext (the only way to work)
- free_secondary_stack not yet tested actively
- could be some problems with alignment/etc related to secondary stack allocation 
- do not restore temporary/etc registers (for amd64 preserve only rbp/rbx/r12/r13/r14/r15/rsi/rdi/rdx/rcx/r8/r9/rip/rsp general registers and fp_state and MXCSR)
- not sure that fpstate stored and restored correctly, not tested yet

> 
> And I still need to patch part of libc related to anonymous memory mapping.
> Updated implementation for libc 21.02:
> 
> https://github.com/tor-m6/genode/commit/c64bfeeba6e8cca4376636bdd2e9e8bbf3aae1cc