Should eventually make C1 thread safe

@prt-linux-q1-1$ run $jp/bin/java_g
VM option '+BackgroundCompilation'
VM option 'CICompilerCount=2'
# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc:  SuppressErrorAt=/c1_Compilation.cpp:500]
#
# An unexpected error has been detected by HotSpot Virtual Machine:
#
#  Internal Error (/BUILD_AREA/jdk6.0/hotspot/src/share/vm/c1/c1_Compilation.cpp, 500), pid=11451, tid=5126
#
# Java VM: Java HotSpot(TM) Client VM (1.6.0-ea-b09-debug mixed mode)
#
# Error: assert(_arena == __null,"shouldn't only one instance of Compilation in existence at a time")
# An error report file with more information is saved as hs_err_pid11451.log
#
# If you would like to submit a bug report, please visit:
#   http://java.sun.com/webapps/bugreport/crash.jsp
#
###@###.### 10/28/04 21:44 GMT