Fix GCC 4.7 warnings, which are related to char being signed in GCC

Fix GCC 4.7 warnings, which are related to char being signed in GCC
("narrowing conversion ... inside { } is ill-formed in C++11").

* src/mksnapshot.cc: Cast "char" to "unsigned char" when outputting snapshot.
* test/cctest/test-regexp.cc: Use static_cast to uc16 as the char
literal is signed.

Review URL: http://codereview.chromium.org/8825003
Patch from Tobias Burnus <burnus@net-b.de>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@10241 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

Change-Id: I180b302d8d38b859b0954553985ea038f27135eb
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>