ARM: Clean up FlushICache.

In theory, the Thumb version of FlushICache doesn't have to be any
different from the ARM version: There is no need to switch to ARM mode
for the svc, and r7 can be used just like any other register.

In practice, GCC uses r7 as a frame pointer in Thumb code, and it
refuses to allow it be used in inline assembly. To avoid complicated
checks to determine whether or not it is possible to use it directly,
the FlushICache function explicitly preserves r7.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/335133002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21949 ce2b1a6d-e550-0410-aec6-3dcde31c8c00