systemc: Add a systemc_home directory which maps to the ext headers.

Some systemc code bases expect to find a SYSTEMC_HOME environment
variable which points to the installed header files provided by
systemc, all under ${SYSTEMC_HOME}/include. The systemc headers in
gem5 are not supposed to be installed anywhere, but to satisfy those
expectations this change creates a dummy systemc_home directory with
an include/ in it which has headers which just include the actual
headers in src/systemc/ext.

More gem5 aware code bases can still access the headers either by
letting gem5's scons environment -I the ext directory, or can do so
themselves if they're not being built by gem5's scons.

Change-Id: I5f2e6bfcf20dd314d525207c2e13ca53474a33f3
Reviewed-on: https://gem5-review.googlesource.com/c/16263
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>