toolchain: glibc: set min kernel version to 6.6

toolchain: glibc: set min kernel version to 6.6

Minimum kernel version is 6.6 so it's safe to increase minimum kernel version to 6.6 for glibc too.
This change effectively removes last compatibility quirk in [1] which leads to better/faster detection of SVE support in string ifuncs for aarch64.
Furthermore, this change will allow to use "fchmodat2" syscall within fchmodat() wrapper in future glibc versions (starting from 2.39) as noted in [2] and [3].

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/aarch64/cpu-features.c;h=4a205a6b35050ffcfac771b4b4ff2d9fb7581f93;hb=refs/heads/release/2.38/master
[2] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/kernel-features.h;h=86b2d3ce512588c8823c11076c91116092836402;hb=refs/heads/release/2.41/master
[3] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=dd1fa5db86bde99fe3eb4804b06c5adf11914b94;hb=refs/heads/release/2.41/master

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18850
Signed-off-by: Robert Marko <robimarko@gmail.com>