selftests: fix kvm relocatable native/cross builds and installs

kvm test Makefile doesn't fully support cross-builds and installs.
UNAME_M = $(shell uname -m) variable is used to define the target
programs and libraries to be built from arch specific sources in
sub-directories.

For cross-builds to work, UNAME_M has to map to ARCH and arch specific
directories and targets in this Makefile.

UNAME_M variable to used to run the compiles pointing to the right arch
directories and build the right targets for these supported architectures.

TEST_GEN_PROGS and LIBKVM are set using UNAME_M variable.
LINUX_TOOL_ARCH_INCLUDE is set using ARCH variable.

x86_64 targets are named to include x86_64 as a suffix and directories
for includes are in x86_64 sub-directory. s390x and aarch64 follow the
same convention. "uname -m" doesn't result in the correct mapping for
s390x and aarch64. Fix it to set UNAME_M correctly for s390x and aarch64
cross-builds.

In addition, Makefile doesn't create arch sub-directories in the case of
relocatable builds and test programs under s390x and x86_64 directories
fail to build. This is a problem for native and cross-builds. Fix it to
create all necessary directories keying off of TEST_GEN_PROGS.

The following use-cases work with this change:

Native x86_64:
make O=/tmp/kselftest -C tools/testing/selftests TARGETS=kvm install \
 INSTALL_PATH=$HOME/x86_64

arm64 cross-build:
make O=$HOME/arm64_build/ ARCH=arm64 HOSTCC=gcc \
	CROSS_COMPILE=aarch64-linux-gnu- defconfig

make O=$HOME/arm64_build/ ARCH=arm64 HOSTCC=gcc \
	CROSS_COMPILE=aarch64-linux-gnu- all

make kselftest-install TARGETS=kvm O=$HOME/arm64_build ARCH=arm64 \
	HOSTCC=gcc CROSS_COMPILE=aarch64-linux-gnu-

s390x cross-build:
make O=$HOME/s390x_build/ ARCH=s390 HOSTCC=gcc \
	CROSS_COMPILE=s390x-linux-gnu- defconfig

make O=$HOME/s390x_build/ ARCH=s390 HOSTCC=gcc \
	CROSS_COMPILE=s390x-linux-gnu- all

make kselftest-install TARGETS=kvm O=$HOME/s390x_build/ ARCH=s390 \
	HOSTCC=gcc CROSS_COMPILE=s390x-linux-gnu- all

No regressions in the following use-cases:
make -C tools/testing/selftests TARGETS=kvm
make kselftest-all TARGETS=kvm

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>