fPIC in x64 environment

Summary:
Check https://github.com/facebook/rocksdb/pull/15 for context.

Apparently [1], we need -fPIC in x64 environments (this is added only in non-fbcode).

In fbcode, I removed -fPIC per @dhruba's suggestion, since it introduces perf regression. I'm not sure what would are the implications of doing that, but looks like it works, and when releasing to the third-party, we're disabling -fPIC either way [2].

Would love a suggestion from someone who knows more about this

[1] http://eli.thegreenplace.net/2011/11/11/position-independent-code-pic-in-shared-libraries-on-x64/
[2] https://our.intern.facebook.com/intern/wiki/index.php/Database/RocksDB/Third_Party

Test Plan: make check works

Reviewers: dhruba, emayanke, kailiu

Reviewed By: dhruba

CC: leveldb, dhruba, reconnect.grayhat

Differential Revision: https://reviews.facebook.net/D14337