riscv: Use __has_include__ to include <asm/syscalls.h> [BZ #24022]

riscv: Use __has_include__ to include <asm/syscalls.h> [BZ #24022]

<asm/syscalls.h> has been removed by

commit 27f8899d6002e11a6e2d995e29b8deab5aa9cc25
Author: David Abdurachmanov <david.abdurachmanov@gmail.com>
Date:   Thu Nov 8 20:02:39 2018 +0100

    riscv: add asm/unistd.h UAPI header

    Marcin Juszkiewicz reported issues while generating syscall table for riscv
    using 4.20-rc1. The patch refactors our unistd.h files to match some other
    architectures.

    - Add asm/unistd.h UAPI header, which has __ARCH_WANT_NEW_STAT only for 64-bit
    - Remove asm/syscalls.h UAPI header and merge to asm/unistd.h
    - Adjust kernel asm/unistd.h

    So now asm/unistd.h UAPI header should show all syscalls for riscv.

<asm/syscalls.h> may be restored by

Subject: [PATCH] riscv: restore asm/syscalls.h UAPI header
Date: Tue, 11 Dec 2018 09:09:35 +0100

UAPI header asm/syscalls.h was merged into UAPI asm/unistd.h header,
which did resolve issue with missing syscalls macros resulting in
glibc (2.28) build failure. It also broke glibc in a different way:
asm/syscalls.h is being used by glibc. I noticed this while doing
Fedora 30/Rawhide mass rebuild.

The patch returns asm/syscalls.h header and incl. it into asm/unistd.h.
I plan to send a patch to glibc to use asm/unistd.h instead of
asm/syscalls.h

In the meantime, we use __has_include__, which was added to GCC 5, to
check if <asm/syscalls.h> exists before including it.  Tested with
build-many-glibcs.py for riscv against kernel 4.19.12 and 4.20-rc7.

	[BZ #24022]
	* sysdeps/unix/sysv/linux/riscv/flush-icache.c: Check if
	<asm/syscalls.h> exists with __has_include__ before including it.

(cherry picked from commit 0b9c84906f653978fb8768c7ebd0ee14a47e662e)