x86-64/EFI: add CFLAGS to check compile

x86-64/EFI: add CFLAGS to check compile

Without this, the compilation of check.c could fail due to compiler
features such as -fstack-protector being enabled, which causes a
missing __stack_chk_fail symbol error.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Rather than using plain CFLAGS here, remove CFLAGS-y from them to
particularly get rid of the -MF argument referencing (the undefined
here) $(@F).

The use of CFLAGS at once allows dropping the explicit use of -Werror.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Committed-by: Jan Beulich <jbeulich@suse.com>