kselftest/arm64: mte: Fix pthread linking

The GCC manual suggests to use -pthread, when linking with the PThread
library, also to add this switch to both the compilation and linking
stages.

Do as the manual says, to fix compilation with Ubuntu's 20.04 toolchain,
which was getting -lpthread too early on the command line:
------------
/usr/bin/ld: /tmp/cc5zbo2A.o: in function `execute_test':
tools/testing/selftests/arm64/mte/check_gcr_el1_cswitch.c:86:
	undefined reference to `pthread_create'
/usr/bin/ld: tools/testing/selftests/arm64/mte/check_gcr_el1_cswitch.c:90:
	undefined reference to `pthread_join'
------------

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-3-andre.przywara@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>