perf: build: Setup PKG_CONFIG_LIBDIR for cross compilation

On recent Linux distros like Ubuntu Noble and Debian Bookworm, the
'pkg-config-aarch64-linux-gnu' package is missing. As a result, the
aarch64-linux-gnu-pkg-config command is not available, which causes
build failures.

When a build passes the environment variables PKG_CONFIG_LIBDIR or
PKG_CONFIG_PATH, like a user uses make command or a build system
(like Yocto, Buildroot, etc) prepares the variables and passes to the
Perf's Makefile, the commit keeps these variables for package
configuration. Otherwise, this commit sets the PKG_CONFIG_LIBDIR
variable to use the Multiarch libs for the cross compilation.

Signed-off-by: Leo Yan <leo.yan@arm.com>
Tested-by: Ian Rogers <irogers@google.com>
Cc: amadio@gentoo.org
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20240717082211.524826-2-leo.yan@arm.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>