package/glibc: fix build on RISC-V with gcc 10.x

package/glibc: fix build on RISC-V with gcc 10.x

Backport upstream commit to use __has_include:
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=28dd3939221ab26c6774097e9596e30d9753f758

Otherwise glibc fails to build with gcc 10:

warning: "__has_include__" is not defined, evaluates to 0 [-Wundef]
../sysdeps/unix/sysv/linux/riscv/flush-icache.c:24:21: error: missing binary operator before token "("
   24 | #if __has_include__ (<asm/syscalls.h>)
      |                     ^

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/655618374

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>