Allow building LD_PRELOAD-able tcmalloc shared library

[vsrinivas]

Certain workloads may wish to use this release of tcmalloc but may
not be easy to rebuild linked against this library. Add a build
target for libtcmalloc.so, which can be LD_PRELOAD-ed.

[ckennelly]

Modifying the BUILD file to no longer have the linkstatic=1 attribute should produce a libtcmalloc.so.

Since I think this PR pulls in all of TCMalloc's dependencies (including Abseil), this could trigger ODR issues for programs that use Abseil and LD_PRELOAD the produced .so.

[vsrinivas]

Going to hold off on the pull request now - it served a simple need I had (LD_PRELOAD-able tcmalloc for use with gn / ninja), but it has fairly tricky ODR concerns as a general solution; we want any libtcmalloc.so target to not 'bake in abseil' for the reason Chris mentioned. And short of having abseil produce an .so, I can't find a clear solution.

[jaidayal]

I commented out linkstatic = 1 in all the BUILD files, but it still never built a libtcmalloc.so.