Build libcompat (lib32) with a --sysroot pointing into its stage directory.

This overrides the cross-compiler's default sysroot to use the WORLD32's
sysroot for building the lib32 libraries. Previously the cross-compiler
would default the sysroot to the 64bit WORLDTMP and -B/-L/-isystem flags
were used to build using the lib32 files. This leads to multiple issues
discussed later. Some extra headers are now needed to be staged since the
64bit WORLDTMP is not referenced at all for headers. The 64bit WORLDTMP
is still used via PATH for build tools. Overriding the default
target/arch is retained in the CC/CXX overrides.

This allows reverting the LDSCRIPT rewriting in installworld from r296921 and
r235122, thus allowing read-only objdirs to work for installing again.

This removes the need for _LDSCRIPTROOT.

This allows progressing the change to always use --sysroot for the build
rather than only relying on the cross-compiler's default sysroot. The
work for that is in D3970 and needed to resolve WITHOUT_CROSS_COMPILER
not using a --sysroot [1].

PR: 196193 [1]
Sponsored by: EMC / Isilon Storage Division