tools: ynl: extend CFLAGS to keep options from environment

Package build environments like Fedora rpmbuild introduced hardening
options (e.g. -pie -Wl,-z,now) by passing a -spec option to CFLAGS
and LDFLAGS.

ynl Makefiles currently override CFLAGS but not LDFLAGS, which leads
to a mismatch and build failure:
        CC sample devlink
  /usr/bin/ld: devlink.o: relocation R_X86_64_32 against symbol `ynl_devlink_family' can not be used when making a PIE object; recompile with -fPIE
  /usr/bin/ld: failed to set dynamic section sizes: bad value
  collect2: error: ld returned 1 exit status

Extend CFLAGS to support hardening options set by build environment.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/265b2d5d3a6d4721a161219f081058ed47dc846a.1731399562.git.jstancek@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>