toolchain: gcc: fix extra space when using SECTION_VAR

The generic version of SECTION_VAR() is adding an extra space
during expansion before second argument (e.g.
SECTION_VAR(section, variable) => ".section. variable"
instead of ".section.variable") which would result in build
error. So remove the "##" in macro to fix this, and now it
will also behave the same as SECTION_FUNC().

Signed-off-by: Daniel Leung <daniel.leung@intel.com>