Change sym_check to filter non-stdlib symbols.

Change sym_check to filter non-stdlib symbols.

Currently sym_check almost all names found in the binary, including those
which are defined in other libraries. This makes our ABI lists harder to maintain.

This patch adds a --only-stdlib-symbols option to sym_check which removes
all symbols which aren't possibly provided by libc++. It also re-generates
the linux ABI list after making this change.

llvm-svn: 287294