perf/Makefile: do not open-code shell-sq

d24e473e (perf_counter: copy in Git's top Makefile, 2009-04-20) started
by determining *_SQ variables (shell-quoted equivalents) by calling
subst by hand, with the rationale that $(call) must be avoided to
accommodate ancient setups.  That reasoning does not hold true anymore,
as our Makefiles are filled with $(call) invocations now.  So, use the
shell-sq function introduced in ced465c4 (perf tools: Makefile:
PYTHON{,_CONFIG} to bandage Python 3 incompatibility, 2011-04-02) in
place of open-coding.

Cc: Michael Witten <mfwitten@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>