Add LLVM GCOV coverage support

With this patch, it is now possible to generate coverage information
for MINIX3 system services with LLVM. In particular, the system can
be built with MKCOVERAGE=yes,

Add LLVM GCOV coverage support

With this patch, it is now possible to generate coverage information
for MINIX3 system services with LLVM. In particular, the system can
be built with MKCOVERAGE=yes, either with a native "make build" or
with crosscompilation. Either way, MKCOVERAGE=yes will build the
MINIX3 system services with coverage profiling support, generating a
.gcno file for each source module. After a reboot it is possible to
obtain runtime coverage data (.gcda files) for individual system
services using gcov-pull(8). The combination of the .gcno and .gcda
files can then be inspected with llvm-cov(1).

For reasons documented in minix.gcov.mk, only system service program
modules are supported for now; system service libraries (libsys etc.)
are not included. Userland programs are not affected by MKCOVERAGE.

The heart of this patch is the libsys code that writes data generated
by the LLVM coverage hooks into a serialized format using the routines
we already had for GCC GCOV. Unfortunately, the new llvm_gcov.c code
is LLVM ABI dependent, and may therefore have to be updated later when
we upgrade LLVM. The current implementation should support all LLVM
versions 3.x with x >= 4.

The rest of this patch is mostly a light cleanup of our existing GCOV
infrastructure, with as most visible change that gcov-pull(8) now
takes a service label string rather than a PID number.

Change-Id: I6de055359d3d2b3f53e426f3fffb17af7877261f

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