[Sanitizers, CMake] Basic sanitizer Solaris support (PR 33274)

Summary:
This patch, on top of https://reviews.llvm.org/D40898, contains the build system
changes necessary to enable the Solaris/x86 sanitizer port.

The only issue of note is the libclang_rt.sancov_{begin, end} libraries: clang relies on the
linker automatically defining __start_SECNAME and __stop_SECNAME labels for
sections whose names are valid C identifiers.  This is a GNU ld extension not present
in the ELF gABI, also implemented by gold and lld, but not by Solaris ld.  To work around
this, I automatically link the sancov_{begin,end} libraries into every executable for now.
There seems to be now way to build individual startup objects like crtbegin.o/crtend.o,
so I've followed the lead of libclang_rt.asan-preinit which also contains just a single
object.

Reviewers: kcc, alekseyshl

Reviewed By: alekseyshl

Subscribers: srhines, kubamracek, mgorny, fedor.sergeev, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40899

llvm-svn: 321373