[PATCH android/bionic 1/3] Hide private symbols using GCC visibility pragma

libc.so should not export all private symbols to reflect Android NDK. This patch attempts to use GCC visibility pragma to hide the private symbols in bionic. It also lets the optimizer produce better and smaller code.

Reference code size measurements for HTC Passion: $ arm-eabi-size out/target/product/passion/system/lib/libc.so

[before] text data bss dec hex filename 260000 10004 45944 315948 4d22c out/target/product/passion/system/lib/libc.so

[after] text data bss dec hex filename 248655 9940 45944 304539 4a59b out/target/product/passion/system/lib/libc.so

Code Review: https://review.source.android.com/#change,17361