toolchain: gcc: make config consistent with glibc/musl

toolchain: gcc: make config consistent with glibc/musl

I've observed configuration drift for GCC between musl and glibc
(especially it's final stage):

  # musl
  lt_cv_prog_compiler_static_works=yes
  lt_cv_prog_compiler_static_works_CXX=yes
  lt_cv_sys_max_cmd_len=1572864

  # glibc
  lt_cv_prog_compiler_static_works=no
  lt_cv_prog_compiler_static_works_CXX=no
  lt_cv_sys_max_cmd_len=512

These changes should prevent this issue in future:

  export lt_cv_prog_compiler_static_works=yes
  export lt_cv_prog_compiler_static_works_CXX=yes
  export lt_cv_sys_max_cmd_len=1572864

Also:

- provide custom autotools/libtool variables via properly named
  variable ("GCC_CONFIGURE_VARS"),
- move variables from "GCC_MAKE" to "GCC_CONFIGURE_VARS"
  (at this moment only "gcc_cv_libc_provides_ssp=yes" for musl),
- propagate it's usage for both "./configure" and "make".

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18646
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>