ar71xx: fix lzma-loader build with glibc

ar71xx: fix lzma-loader build with glibc

For an unknown reason gcc tries to link in crti.o when building with a
glibc toolchain (this does not happen with other targets). Prevent this
by telling gcc explicitly to not do that.

Fixes the following build error:

/home/jonas/git/lede/staging_dir/toolchain-mips_24kc_gcc-5.5.0_glibc/lib/gcc/mips-openwrt-linux-gnu/5.5.0/../../../../mips-openwrt-linux-gnu/lib/crti.o: In function `_init':
(.init+0x18): relocation truncated to fit: R_MIPS_GOT16 against `__gmon_start__'
/home/jonas/git/lede/staging_dir/toolchain-mips_24kc_gcc-5.5.0_glibc/lib/gcc/mips-openwrt-linux-gnu/5.5.0/../../../../mips-openwrt-linux-gnu/lib/crti.o: In function `_init':
(.init+0x28): relocation truncated to fit: R_MIPS_CALL16 against `__gmon_start__'
collect2: error: ld returned 1 exit status

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>