NFC: Switch ATRACE_BUFFER_INDEX to be a more friendly macro statement.

In the process of upgrading to clang r349610, a new diagnostic,
-Wextra-semi-stmt, enabled by -Weverything catches redundant semicolons
in code.  ATRACE_BUFFER_INDEX is a macro statement (currently using an if
with a code block that ends in a '}'). Since most callers are treating
this as a pseudo-function call, they terminate the macro with a
semicolon. This triggers the diagnostic, as we now have "if (...) { ...
};", where the semicolon is unnecessary.

To remedy these kinds of situations, it is better to construct macro
statements that require a semicolon to terminate them. This patch uses a
do/while wrapped around the existing statement, which is a pretty common
pattern for macro statements.

Bug: http://b/122481018
Test: m checkbuild

Change-Id: Ib1271e8634b0a6bd05b27d78ae17f63feb0e5f65