[11u] RFR 8254790: SIGSEGV in string_indexof_char and stringL_indexof_char intrinsics

[Viswanathan, Sandhya]

The fix for 8254790 needs to be backported to JDK11u.
The fix is one-line change in string_indexof_char intrinsic.
The intrinsic is moved to c2_MacroAssembler_x86.cpp since jdk15 and was in macroAssembler_x86.cpp in JDK 11u so I am sending a separate webrev review request.
The other stringL_indexof_char intrinsic didn't exist in JDK11u so that fix is omitted in this webrev.

JBS: https://bugs.openjdk.java.net/browse/JDK-8254790
Webrev: http://cr.openjdk.java.net/~sviswanathan/8254790/webrev.00/

Best Regards,
Sandhya