Add build/cross-build dependency check script kselftest_deps.sh
This script does the following:

Usage: ./kselftest_deps.sh -[p] <compiler> [test_name]

	kselftest_deps.sh [-p] gcc
	kselftest_deps.sh [-p] gcc vm
	kselftest_deps.sh [-p] aarch64-linux-gnu-gcc
	kselftest_deps.sh [-p] aarch64-linux-gnu-gcc vm

- Should be run in selftests directory in the kernel repo.
- Checks if Kselftests can be built/cross-built on a system.
- Parses all test/sub-test Makefile to find library dependencies.
- Runs compile test on a trivial C file with LDLIBS specified
  in the test Makefiles to identify missing library dependencies.
- Prints suggested target list for a system filtering out tests
  failed the build dependency check from the TARGETS in Selftests
  the main Makefile when optional -p is specified.
- Prints pass/fail dependency check for each tests/sub-test.
- Prints pass/fail targets and libraries.
- Default: runs dependency checks on all tests.
- Optional test name can be specified to check dependencies for it.

To make LDLIBS parsing easier
- change gpio and memfd Makefiles to use the same temporary variable used
  to find and add libraries to LDLIBS.
- simlify LDLIBS append logic in intel_pstate/Makefile.

Results from run on x86_64 system (trimmed detailed pass/fail list):
========================================================
Kselftest Dependency Check for [./kselftest_deps.sh gcc ] results...
========================================================
Checked tests defining LDLIBS dependencies
--------------------------------------------------------
Total tests with Dependencies:
55 Pass: 53 Fail: 2
--------------------------------------------------------
Targets passed build dependency check on system:
bpf capabilities filesystems futex gpio intel_pstate membarrier memfd
mqueue net powerpc ptp rseq rtc safesetid timens timers vDSO vm
--------------------------------------------------------
FAIL: netfilter/Makefile dependency check: -lmnl
FAIL: gpio/Makefile dependency check: -lmount
--------------------------------------------------------
Targets failed build dependency check on system:
gpio netfilter
--------------------------------------------------------
Missing libraries system
-lmnl -lmount
--------------------------------------------------------
========================================================

Results from run on x86_64 system with aarch64-linux-gnu-gcc:
(trimmed detailed pass/fail list):
========================================================
Kselftest Dependency Check for [./kselftest_deps.sh aarch64-linux-gnu-gcc ]
results...
========================================================
Checked tests defining LDLIBS dependencies
--------------------------------------------------------
Total tests with Dependencies:
55 Pass: 41 Fail: 14
--------------------------------------------------------
Targets failed build dependency check on system:
bpf capabilities filesystems futex gpio intel_pstate membarrier memfd
mqueue net powerpc ptp rseq rtc timens timers vDSO vm
--------------------------------------------------------
--------------------------------------------------------
Targets failed build dependency check on system:
bpf capabilities gpio memfd mqueue net netfilter safesetid vm
--------------------------------------------------------
Missing libraries system
-lcap -lcap-ng -lelf -lfuse -lmnl -lmount -lnuma -lpopt -lz
--------------------------------------------------------
========================================================
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>