libucontext: bump to 1.5.1 & glibc support

libucontext: bump to 1.5.1 & glibc support

Changelog:
Changes from 1.5 to 1.5.1
-------------------------
* Rename public register definitions on x86 and x86_64 to avoid clashing
  with glibc definitions when built in freestanding mode.
  Patch contributed by Daan De Meyer.

Changes from 1.4 to 1.5
-----------------------

* Workaround various header compatibility issues in glibc and bionic.

* Allow ppc64 target to be built in freestanding mode.

Changes from 1.3.3 to 1.4
-------------------------

* Add hard-float support to most architectures where relevant.
  Some patches contributed by Richard Campbell.

Changes from 1.3.2 to 1.3.3
---------------------------

* Use `bl` instruction instead of explicit GOT lookup for `exit`
  in 64-bit PowerPC assembly.

* Fix installation path for standalone libucontext headers.
  Patch contributed by Marian Buschsieweke.

* Fix missing includes for freestanding builds.
  Patch contributed by David Leeds.

Makefile:
  Disabled documents.
  Added glibc supports, and remove hardcoded musl dependency.
  Put test_libucontext_posix only available in musl runtime.

Build system: x86/64
Build-tested: x86/64-glibc
Build-tested: x86/64-musl
Run-tested: x86/64-glibc

Signed-off-by: Aditya Nugraha <redacted>